The Mur'gul was a unit that appeared as part of the Naga forces and as creeps in certain maps in Warcraft 3 The Frozen Throne.
Their orginal lore tidbit described them as Murlocs corrupted by the Burning Legion.
This was never further explored or explained and the Mur'gul didn't make any other apperances in the other games.
Because of their absence in 8,2, I decided to give them a go with a little change: instead of the Burning Legion, what if they were changed by N'zoth.