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Scenarios › Isles of Abyss › The Sunken Lord

Once Human Sunken Lord: How to Unlock & Beat the Final Boss

LiveUpdated 2026-10-23 · Game v3.1.1Phase II endgame · unlocks server-wide

By the Once Human Guide crew — a small fan team led by Yao · from patch notes + videos

Short answer: the Sunken Lord opens in Phase II, once your server’s purification progress reaches 100% and its entrance platform rises out of the sea. Beat the Calamity Hatchling at the entrance, win the gate battle, push through the corridors and platforms, then fight the Sunken Lord’s Core. Team size isn’t in the live patch notes: an August test post said 4 players, so plan for 4 until it’s confirmed.

On this page
  1. How the Sunken Lord unlocks
  2. Team size: the sources disagree
  3. How the fight works
  4. Before you go in
  5. Rewards
  6. More for the endgame
  7. FAQ
  8. What changed

How the Sunken Lord unlocks

The Sunken Lord is the last challenge of the Isles of Abyss scenario, and nobody unlocks it alone. It is tied to your server’s purification progress (also shown as World Progress), which every player on the server pushes together.

  1. Phase I lasts about 5 to 10 days; the faster the server purifies, the shorter it is. Taking down Calamity Towers is one of the main ways to push it, and a first kill also sends the whole server a reward mail. See Calamity Tower & Calamity Nest.

    src: Isles of Abyss dev blog (2026-10-16) · final-test recruitment post (2026-08-28)

  2. Phase II lasts 11 days. Purification keeps rising, and new Phase II content (the Sea Calamity Zone, Voyager fleets, harder towers) appears at sea.

  3. The entrance platform comes up out of the sea. That is the signal that the Sunken Lord is open on your server.

    src: patch notes (2026-10-21) · dev blog (2026-10-16)

The exact day depends on your server. Working from the phase lengths, most servers should reach it in early to mid November estimate, not an official date. Scenario dates and phases are on the Isles of Abyss hub.

Team size: the sources disagree

This is the one thing to check before you build a group around it.

28 Aug
The official final-test recruitment post describes the Sunken Lord as a 4-player dungeon.
16 & 21 Oct
The launch dev blog and the live patch notes describe the fight but give no player count.
Creator report
One creator says an earlier pioneer test in China ran it for 8 players and the final test for 4, and warns not to treat either number as settled. from one creator’s video, 16 Oct
Early servers
A sponsored early-access video and a test-server guide both call it a four-player fight. pre-launch footage

Our reading: probably 4, but not confirmed for the live servers. We’ll update this line when the developer or first clears settle it.

src: final-test recruitment post (2026-08-28) · EUTOPIA lYn8rzveH-M @05:40 · Open World Games 1_YBnzhRRZ8 @10:00 · Gotcha Beast KJvFq__cWp8 @14:22

How the fight works

The official notes describe it as one run in stages. Damage numbers and HP aren’t published.

  1. The group first defeats the Calamity Hatchling waiting at the entrance.

  2. Finish the battle at the gate to get inside.

  3. Push deeper through corridors and stacked platforms where enemies and tentacles cover for each other. Clear them as a group rather than spreading out.

  4. The last stage is the Core itself, which changes as the fight goes on.

    src: patch notes (2026-10-21) · dev blog (2026-10-16)

Attacks to expect

  • Tentacle sieges around the arena.
  • Projectile barrages in patterns.
  • Hull smashes: it swings broken ship hulls into the floor.
  • A shrinking safe space. The live patch notes say spreading slime and illusions close in on you. The October preview described lava instead; go by the patch notes.

One sponsored early-access video shows the boss as a giant crab-like creature, with the final part of the fight taking place inside its heart. pre-launch footage

src: patch notes (2026-10-21) · dev blog (2026-10-16) · Open World Games 1_YBnzhRRZ8 @10:00

Before you go in

  • Bring Heart of the Ocean. You spend it at the reward stage after the fight, so save some from challenges, Voyagers, crates and treasure.
  • Gear up through Tech. Gear rarity and active mod attributes are capped by your Tech level, so the run goes better once you’ve unlocked the later tiers.
  • Prepare for heat and cold on the way. A test-server guide warns that the fourth and fifth Haywire seas have islands with extreme temperatures and suggests running All-Weather Stew from your base. test-server advice Counters for each weather: Haywire zones.

src: patch notes (2026-10-21) · dev blog (2026-10-16) · Gotcha Beast KJvFq__cWp8 @14:22

Rewards

reward how you get it notes
Duet (new Deviation) chance to drop from the Sunken Lord changes the weather effects in your territory in real time
Precision Load (Armor Mod, lower body) Sunken Lord, or spend Heart of the Ocean at the reward stage returns bullets after long-range weak-spot hits
Turret Enhancement (Armor Mod, gloves) Sunken Lord, or spend Heart of the Ocean at the reward stage raises damage from facilities you operate, such as ship cannons
Shiny mod box first clear from one pre-launch video not in the patch notes

Drop rates aren’t published. For gunners, Turret Enhancement pairs with the cannons in Ship cannons.

src: patch notes (2026-10-21) · Open World Games 1_YBnzhRRZ8 @11:40

More for the endgame

FAQ

When does the Sunken Lord unlock?
When your server’s purification progress reaches 100%, which happens during Phase II. There is no fixed date: Phase I lasts about 5–10 days depending on purification, and Phase II lasts 11 days, so on most servers it should fall somewhere in early to mid November. That window is an estimate from the phase lengths, not an official date.
Is the Sunken Lord a 4-player or 8-player fight?
Not confirmed for the live servers. The official final-test recruitment post (28 Aug) called it a 4-player dungeon; the 21 Oct patch notes give no number; one creator reports that an earlier test in China ran it for 8. Plan for 4 and check again once servers reach it.
Can I solo the Sunken Lord?
Not confirmed yet. No official source mentions a solo mode for this fight.
What does the Sunken Lord drop?
The new Deviation Duet has a chance to drop, and the reward stage lets you spend Heart of the Ocean on rewards such as the new Precision Load and Turret Enhancement mods. Drop rates are not published.
Where is the Sunken Lord on the map?
The official notes don’t give a location; they only say the entrance platform rises from the water at 100% purification. This page will add it once it’s known.

What changed

  • v3.1.1 · 2026-10-23Page created.

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