records/endlessdungeon/creatures/enemies/shatteredboss/cultist_chthonianmonstrosity.dbr shows Thall should use this bio file records/endlessdungeon/creatures/enemies/bios/bio_boss_shatteredbossp2_01.dbr. And its health formula should be ((charLevel*46)^1.54)+12000. For ultimate SR 31, it is about 9 million.
But actually, Thall has about 18 million health in SR 31. So here is my guess. It may mistakenly use the main compaign data file records/creatures/enemies/bios/bio_boss_chthonianabomination_02.dbr and health formula ((charLevel*70)^1.54)+22000, which is about 18 million.
Further more, Karroz, Sigil of Ch’thon may be the main compaign version, as records/endlessdungeon/proxies/poolsboss/chthonian_karroz.dbr spawn this version records/creatures/enemies/boss&quest/cultist_summoner_01.dbr, instead of the SR version records/endlessdungeon/creatures/enemies/shatteredboss/cultist_summoner_01.dbr.

