HARROWING ÆTHILLU: UMBRAL MONUMENT STAGE 5

Harrowing Æthillu is Stage 5 of the Contention of Deathly Silences series in Arknights: Endfield’s permanent endgame mode, Umbral Monument. It arrived in Version 1.3 on July 2, 2026, and the fight is built around one enemy with one nasty trick: Æthillu revives after you kill it, comes back with longer reach, and can detonate itself for a huge AoE hit. There is no elemental counter here. Æthillu resists Physical, Heat, Electric, Cryo, and Nature all at exactly the same rate, so the stage is a pure burst-and-timing test rather than a matchup puzzle. If you have been staring at a boss that refuses to stay dead, this guide walks through the revival mechanic, the two-second kill window, the teams that clear it, and the lore behind the phantom.

TL;DR - Key Points

• Harrowing Æthillu is Stage 5 of Contention of Deathly Silences, live since July 2, 2026, and permanently available
• Æthillu revives after death with extended attack range and the option to self-detonate for massive AoE damage
• No element counters it - Physical, Heat, Electric, Cryo, and Nature all deal 0.8x damage; only Ether hits for 1.0x, and no standard operator uses Ether
• Bring your single strongest, highest-burst four-operator team, not an “off-element” squad
• Stagger HP is only 80 (6-second recovery), so staggers come fast; land a Finisher in the window for a burst spike and 25 SP back
• Hold a nuke for the revival and delete the revived form within about 2 seconds, which also earns the “Unstable Æthillu” medal
• In Agony mode, consumables are banned, so all sustain must come from operator kits
• Some Stage 5-specific details (the exact unique modifier) are flagged unconfirmed and will be updated

Where Harrowing Æthillu Fits in the Series

Contention of Deathly Silences is the current Umbral Monument series, and it is one of the larger ones, shipping with six stages. It launched July 2, 2026 and stays permanently available afterward, so there is no rush on the stage itself. Only the launch-window event has a deadline. The full stage lineup:

A design rule that holds across this entire series is worth internalizing before you build a team: enemies share the same resistance to every element. That turns each stage into a test of positioning, mechanics, and raw damage output instead of elemental rock-paper-scissors. Stage 5 leans into that harder than most, because Æthillu’s revival makes timing, not typing, the whole game.

What Is the Umbral Monument?

Umbral Monument is Endfield’s first permanent, high-difficulty endgame combat mode, and it plays the same role Contingency Contract does in the original Arknights. It first launched February 12, 2026 with the debut series “Those Forsaken by the Land,” and new series have rotated in roughly every six to eight weeks since. Past series stay playable forever; only their bonus reward windows expire.

Here is how each stage is structured:

• Two difficulties. Normal mode (recommended Lv. 60) and Agony mode (recommended Lv. 90). You must clear Normal to unlock Agony. Agony hits harder, adds modifiers, and bans all Tacticals and consumables, so no item heals or buffs.
• Four operators, ten-minute timer. Each fight caps you at four deployed operators and a 10-minute clock. The timer is a fail condition only; clearing faster does not change rewards.
• Series medals. Clearing every stage in Normal earns an Engraved Medal; clearing every stage in Agony trims it. That trimmed medal is the endgame flex.

How to unlock the mode: complete the main mission Chapter I Process II: Paving the Way and reach Authority Level 30, then talk to Sergio Leon at the Valley IV Hub Base.

For a cross-series breakdown of every stage and enemy so far, the Umbral Monument Database tracks the lot.

Who, or What, Is Æthillu?

Æthillu (also written Aethillu; internal name eny_0095_ethillu) is a Common enemy of the Wildlife class. Its localized names all circle the same idea of a lingering echo: 表象 in Chinese (“appearance”), イーサルー in Japanese, 잔영 in Korean (“afterimage”), Éthillu in Spanish, and Эфирлюзия in Russian. Its in-game description reads: “A phantom that answered Ardashir’s call. It is a manifestation of destruction, a sedimentation of ancient senescence.”

That is a lot of lore packed into one line, and we unpack it near the end. First, the numbers that decide the fight.

Æthillu combat profile (verified)

Two things jump out. First, the uniform 0.8x resistance means there is no elemental advantage to chase. Every team you can build eats the same penalty, so the correct answer is your best-geared squad, full stop. Second, that low Stagger HP of 80 is a gift: you can stagger Æthillu quickly and often, which is the setup for the burst plan below.

The revival mechanic, the heart of the fight

Æthillu’s signature ability, verbatim: “This enemy has a powerful revival ability. The revived form has a longer attack range. It can also detonate itself to deal massive DMG.”

So Æthillu does not stay dead. When you drop it, it revives, returns with extended attack range, and can choose to self-destruct for a large AoE burst. This single mechanic is what the whole stage is built around, and it is why Endfield ships a dedicated combat medal, “Unstable Æthillu,” whose condition is to “defeat an ‘Æthillu’ within 2 seconds of its revival.”

The takeaway writes itself: the intended counter is speed. You want enough saved burst to erase the revived form almost instantly, before it can extend its reach or line up a detonation.

How to Beat Harrowing Æthillu, Step by Step

Verification note: the mode-wide rules, Æthillu’s mechanics, and the series template are fully verified. The Stage 5-specific unique modifier and its exact enemy roster come from an in-game page we could not open at publication, so anything unique to Stage 5 is flagged unconfirmed and will be updated. Treat this as a mechanically-grounded playbook, not a patch-note transcript.

1. Build for burst, not for element. Because Æthillu resists all five elements equally, do not scramble for an off-element team. Bring your single strongest, best-geared four-operator squad, ideally the one with the fastest path to Stagger and the biggest Finisher or ultimate payload.

2. Stagger, then Finisher. Æthillu’s Stagger HP is only 80, but it recovers in six seconds, so you have a window, not forever. Stack Stagger fast with Final Strikes, Battle Skills, Combo Skills, and ultimates, then land your Finisher inside the Stagger window for a burst spike and 25 SP back.

3. Save a nuke for the revival window. The revive is both the danger point and the scoring point. Hold a burst cooldown, an ultimate or a Combo Skill, so that the instant Æthillu revives you can delete it within roughly two seconds. That is exactly what the “Unstable Æthillu” medal rewards, and it also denies the revived form the chance to extend its range or set up a detonation.

4. Respect the detonation. If you cannot kill the revived form immediately, create distance. The self-detonation is a large AoE. Do not let your whole squad cluster inside it.

5. In Agony, sustain through operators only. Agony bans consumables, so survivability has to come from kits. A dedicated healer such as Ardelia or a strong shielder moves from “nice” to close to mandatory.

Recommended Teams

Because the stage is a burst-and-timing check, the strongest Version 1.3 cores across Contention of Deathly Silences are your best starting points. The comps below are the series’ strong general teams and the ones recommended on the sibling Stage 3 guide. They are unconfirmed for Stage 5 specifically, but the “high Stagger, big Finisher” profile lines up cleanly with a burst-the-revival plan.

For pairing logic on the Physical core, our Mi Fu best teams breakdown covers who to slot and who to skip. If you are leaning Electric, the Zhuang Fangyi Umbral team and Ardelia rotation guide walks the exact skill order. And if you are running Ember in the Physical comp, her damage soak and ult timing are detailed in the Ember build guide.

Recommended levels and gear

• Normal mode: operators around Lv. 60 with a coherent gear set.
• Agony mode: a fully built Lv. 90 team you are comfortable piloting, with sustain baked into the kit because items are banned. For the Heat comp, Camille wants Frontiers for combo-cooldown SP generation, explained in the Camille gear sets guide.

Playing by Squad Type

Not every account walks in with the same roster, so here is how the fight changes depending on what you have built.

Whale / heavily-invested accounts. You likely delete the base form and the revived form fast enough that timing barely matters. Just make sure your biggest cooldown is available when Æthillu revives so you also bank the “Unstable Æthillu” medal on the way through.

Mid-invested accounts. This is where the timing discipline pays off most. Your damage is enough to kill, but not enough to ignore the revival. Bank a nuke, stagger for the Finisher, and treat the revive as a scheduled event you have prepped for.

Newer / lower-investment accounts. Clear Normal first for the shared first-clear rewards, then decide whether Agony is worth the retries. Without consumables in Agony, a lower-damage squad will need a real healer or shielder to survive the detonation, so prioritize sustain over a marginal DPS slot.

Common Mistakes to Avoid

• Chasing an elemental counter. There is not one. Uniform 0.8x resistance means your most-built team wins, so stop rerolling comps by element.
• Dumping all your burst before the revival. If your nuke is on cooldown when Æthillu revives, you miss the two-second kill window and eat a detonation.
• Facetanking the revived form. Its extended range punishes lazy positioning. Reposition if you cannot burst it down.
• Bringing a consumable-dependent team into Agony. No Tacticals means no item heals. Plan sustain around operator kits.
• Ignoring Stagger. Skipping Stagger setup slows every kill and drags you toward the ten-minute wall.

Lore: The Phantom That Answered Ardashir’s Call

Æthillu’s flavor text ties it directly to Ardashir, one of Endfield’s primary antagonists. Ardashir is a non-playable character introduced in Wuling and allied with Nefarith. He presents himself as a traveler but is revealed to be a “Pasha,” a high-ranking lord of Sargon, described in-game as “a land of gold and windswept sands from the other side of the Cosmic Gate.” He commands teleportation Arts, summons Æther-related monsters, and is chasing “The Foundation,” or the “Exiles,” from beyond the Æther.

Æthillu, described as “a manifestation of destruction, a sedimentation of ancient senescence,” reads as one of those summoned Æther-phantoms: less a living beast than a lingering afterimage of decay given form. Its Chinese and Korean names (表象, “appearance”; 잔영, “afterimage”) reinforce the theme, and the “Æthillu” name itself evokes the Æther that Ardashir is hunting. In world terms, the creature’s revive-and-detonate cycle is a fitting metaphor: a phantom that cannot truly be killed because it is already an echo of something long gone. The series title, “Contention of Deathly Silences,” and the launch event, “Dirge of Grief,” lean into the same funerary, memorial tone that defines Umbral Monument as a whole.

Rewards and Why You Should Clear It

Clearing Harrowing Æthillu contributes to the series completion rewards, which are identical across stages and difficulties: Mark of Perseverance, Advanced Cognitive Carrier, Arms INSP Set, Protohedron, and T-Creds. It also counts toward the Engraved Medal (all Normal) or its trimmed version (all Agony). During the launch window, clears feed the Monumental Etching: Dirge of Grief event for bonus Oroberyl, the premium pull currency. And a fast burst-kill nets the “Unstable Æthillu” medal.

For the confirmed event window and what its deadline signals about Version 1.4, see our Dirge of Grief day-one breakdown.

Watch List: What Would Change This Guide

A few things could shift the strategy as more data comes in, and this post will be updated when they do:

• The confirmed Stage 5 unique modifier. Every Agony stage layers an extra rule on top. Once the in-game modifier text is documented, it may reshape positioning or which sustain unit you bring.
• The full Stage 5 enemy roster. If Æthillu shares the stage with adds, the “burst the revival” plan has to account for aggro and AoE during the kill window.
• Confirmed team clears. The comps here are drawn from the series meta. As players post verified Stage 5 clears, we will swap in the ones that reliably hit the two-second revival window.

Final Read

Harrowing Æthillu is not a DPS wall so much as a timing wall wearing a DPS costume. There is no element to counter and no gimmick to solve; there is a phantom that revives with longer reach and a self-destruct, and a two-second window where a saved nuke turns the whole fight around. Bring your strongest team, stagger for the Finisher, and hold one big cooldown for the revive. Do that, and the medal and the clear arrive in the same swing. Because the series is permanent, take your time learning the rhythm on Normal before committing your best Lv. 90 squad to Agony’s no-consumables run.

FAQ

What is Harrowing Æthillu in Arknights: Endfield? It is Stage 5 of the Contention of Deathly Silences series in the Umbral Monument endgame mode, a boss-style fight against the phantom enemy Æthillu. It launched in Version 1.3 on July 2, 2026.

How do you kill Æthillu permanently? Æthillu revives after dying, returning with longer range and a self-detonation. The intended counter is speed: stagger it, open a Finisher, and burst the revived form down within about two seconds of its revival, the same condition that awards the “Unstable Æthillu” medal. The precise permanent-kill trigger specific to this stage is unconfirmed at publication and will be updated.

What element counters Æthillu? None. Æthillu resists Physical, Heat, Electric, Cryo, and Nature all at exactly 0.8x damage, so there is no elemental advantage. Bring your strongest, highest-burst team instead.

What is the best team for Harrowing Æthillu? A high-burst, high-Stagger comp. Strong options include Mono-Physical (Mi Fu / Pogranichnik / Chen Qianyu / Ember), Zhuang Fangyi Electric, a Rossi hybrid, and Laevatain Heat with Ardelia for sustain in Agony. These are drawn from the current series meta; stage-5-specific comps are unconfirmed.

What level should my operators be? Around Lv. 60 for Normal mode and a fully built Lv. 90 team for Agony mode, which also bans all consumables.

Is Umbral Monument permanent or limited-time? The stages are permanent. Only the Monumental Etching bonus event, Dirge of Grief for this series, is time-limited.

How do I unlock Umbral Monument? Complete the main mission Chapter I Process II: Paving the Way and reach Authority Level 30, then talk to Sergio Leon at the Valley IV Hub Base.

What is the “Unstable Æthillu” medal? A combat achievement earned by defeating an Æthillu within two seconds of its revival, which lines up perfectly with the burst-the-revival strategy.

Why does Æthillu have only 100 DEF but so much HP? Its flat DEF of 100 stays constant at every level while HP and ATK scale hard. That means percentage-based and high-hit-count damage stays effective at all levels; the wall is its HP pool and revival, not its defense.

Does Agony mode change the Æthillu fight much? Yes. Agony bans all consumables and layers on extra modifiers, so your survivability has to come entirely from operator kits. A dedicated healer or shielder becomes close to mandatory for the detonation.

This guide will be updated as the Stage 5-specific modifier and confirmed team clears are verified. Contention of Deathly Silences is permanently available, so there is no rush on the stage itself, only on the Dirge of Grief event tied to the version window.