Absolum, the true final boss
If you searched final boss, read this first: a normal ascent ends on Azra, the Sun King. The Underking is only Grandery. The creature called Absolum is the extra fight at the tower after you have beaten Azra once, finished another full run, and kept at least one Mother’s Grace.
Only this subject
Walkthroughs describe an opening scripted blow, four drones around 40 percent health that can be used as hazards, a time stop near 1 percent that refills the fight and shifts the scene, and a faster last phase that smashes platforms after the halfway mark of that phase. Tentacles can be Clashed. The tide attack wants a jump. Drones are where ultimates and Arcana get recommended. I treat 40 percent and 1 percent as that walkthrough’s thresholds, not as a datamine.
The ending text is that you help the creature hatch and it leaves Talamh. The achievement is Sleep, my Sweet Prince. The cue cited is Absolum, A Broken Dream, at 4 minutes 15 seconds. Total health is not published. Spoilers above are the reason the ending page exists as the shorter door.
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Sources
Checked 2026-11-07. Applicable public version notes are in the tables above.