In Baldur's Gate 3, General Ketheric Thorm is introduced to players formally around the middle of Act 2, once traversal of the deeper Shadow Cursed lands becomes available. The invulnerable General of the Absolute isn't so tough once players rescue the Nightsong from the Shadowfell, cutting off his source of immortal powers.
Baldur's Gate 3: Ketheric Thorm Boss Guide
Ketheric Thorm, the invulnerable General of the Absolute's armies, is the final boss of Act Two, and he's no pushover – especially in Phase Two.