An undead priest with the Absolutes, Balthazar is a boss in Baldur's Gate 3 you can try to beat at two different points, summoning undead alongside powerful spells that are incredibly dangerous against your party. Like many opponents in this massive RPG, where you fight Balthazar depends on your character's choices. However, once you do, it'll take strong cooperation in your party to take him down.
https://screenrant.com/how-to-beat-balthazar-baldurs-gate-3/