Ras Nsi

Ras Nsi was a powerful Paladin defender of the ancient Chultan city of Mezro.
For unknown reasons, Ras broke his sacred oaths and turned his divine given powers to necromancy. He raised a might undead army that followed his commands and left Mezro for their greatest rival city Omu.
Once Ras Nsi established himself in Omu, he persuaded the cult like followers of Dendar, The Night Serpent and the Yuan-Ti who populated Omu to follow Ras into battle against his former city of Mezro.
The forces of Mezro and warriors from the fledgling coastal settlement of Port Nyanzaru, combined their forces to repel the snake people of Omu and Ras Nsi's undead army. After the battle Ras Nsi disappeared. It is unknown if he was killed in the fight or fled, but he was never seen again in the last 100 years.
Scattered remains of Ras Nsi's undead army still plague the jungles of chult to this day, although the undead now lack any organization or force of will driving them, unlike when they were under the direct command of the Dark Paladin.