In life, N'Gasta was known as the most nefarious Sload Necromancer throughout the whole of Nirn. He crafted the Thrassian Plague that ravaged Tamriel and wiped out large numbers of the populace.
Thousands of years after his death, he has been revived by Arawn and now serves the purposes of the Forgotten Daedric Prince. The Thrassian Plague has been brought back as well, and among other diseases and afflictions, he plagues Valenwood to finalize the efforts of the Thalmor and their Bosmer purges, among other efforts.