Halphas is a Sunken Palace boss. It's guarding the final armor piece.
Strategy[]
The boss has grapple moves, it will grapple in any direction, when on ground it can kneel and use tentacle attack. It is invincible and becomes vulnerable only when it does animation that shows its tentacles.
Weakness[]
After it uses tentacle grapple, it has chance to perform animation where it will expose it weakpoint. You can start attacking it as soon as animation starts. Try to go where boss will land.
Gallery[]
Halphas vulnerable state
Halphas' remains