Rook normally doesn`t take any damage unless stunned but it can take damage from toxic pools created by molded grave gear. It can lead to weird scenarios where he can enter the next phase when earlier phase crystals aren`t destroyed. Also the animation is glitchy. Then killing a crystal from previous phase before the new ones spawn will stun him again.
PS: for some reason i don`t have a player log file from rotwood so i`m attaching the one from ONI so i can report the bug.
Steps to Reproduce
Equip a molded grave gearpiece that can spawn toxic pools (easiest with the one that does it on dodges)
leave a pool next to rook.
Rook takes damage even though it`s supposed to be invulnerable.