Ancient Beast - dungeon, phases and strategy

The Ancient Beast is a level 3000 dungeon raid boss. It awaits you on the 5th floor and is generally the first boss of the run, regardless of difficulty chosen. It operates in two phases, similar to rip_indra True Form: first a humanoid NPC that uses M1 and T-Rex fruit abilities, then, around 30% health, a transformation into T-Rex that restores all its health and unlocks its transformed abilities. Its room is also the only one in the dungeon to have a terrain hazard: from the second phase onward, lava rises in waves.

How to get it

No summon to prepare: you encounter it automatically on the 5th floor of a dungeon, at any difficulty.

Its stats at a glance

Info	Value
Level	3000
Health	285,000 / 615,000 / 915,000
Aura	No
Weapon	Fruit
Location	Dungeon, 5th floor

The source provides three health values without specifying which difficulty each corresponds to: consider them as a range, from easiest to hardest run.

Its two phases

Humanoid Phase. It's a standard NPC that fights using M1 and T-Rex fruit abilities.
T-Rex Phase. Around 30% health, it transforms, restores to full health and unlocks the transformed versions of its attacks, which are wider and more dangerous.

So expect a fight much longer than its initial health bar suggests: transitioning to phase 2 resets the counter to zero.

Its room and the lava

The room is circular, with mostly flat ground, trees, rocks, and large platforms resting on tall stone pillars.

It's the only room in the dungeon with a terrain hazard: from the second phase onward, it periodically fills with lava. Damage increases with dungeon difficulty and becomes extreme if you stay in it too long. Use the high platforms to let the waves pass.

Its attacks

Attack	What it does	How to deal with it
Normal Attack	He puts continuous pressure on you with his M1s throughout the fight.	Stay out of range and keep Instinct active.
Tail Swipe	He sweeps in front of him with his tail, at random. Transformed, the sweep is wider and chains more hits.	Stay out of range. The transformed version's hits only go diagonally: you can camp above him.
Predatory Screech	He charges in a marked cone in front of him, pulls in players, then launches them with a brutal scream. Transformed, the attack deals much more damage, with a noticeably larger hitbox.	Stay out of range. If hit, the scream is Instinct-trickable. Beware the transformed version: it breaks Instinct.
Hunter's Rage	He quickly aligns on a player, charges for a moment, then rushes forward with a flurry of hits. Transformed, he leaps instead and slams you to the ground causing a nausea effect.	Stay out of range. Both versions are Instinct-trickable.
Gigantic Leap	He leaps toward a player to close the distance.	Stay alert, he returns to close range quickly.

Good to know

It does transform into T-Rex, but it doesn't trigger the fruit's Prey Mark passive.
It doesn't use Aura.
It's the first of the dungeon bosses presented by the source, alongside Kitsune Lord, Gas Knight, and Zero.