Lich

To gain more time to complete their goals, some desperate spellcasters pursue immortality by embracing undeath. After long years of research and the creation of a special container called a soul cage, a spellcaster takes the final step by imbibing a deadly concoction or casting dreadful incantations that transform them into a lich.

Lich (Creature 12), Lich Legion (Creature 18)

After their metamorphosis, a lich often finds some quiet place to dwell, typically protected by a variety of guardians and traps, for two primary purposes. First, a lich requires solitude to plan their elaborate schemes, and second, few mortals (if any) deign to interact with these legendarily corrupt necromancers.