Malyk was an aspect of Talos created in the wake of the Time of Troubles to take advantage of areas of wild magic, created after Mystra's death at the hands of Helm. Due to the nature of his followers though, these were usually restricted to areas in the Underdark.
Malyk was a drow lich of increasing power who lived in the Underdark beneath the Dalelands. During the Time of Troubles - after Mystra's death but before Midnight's apotheosis - he came across a powerful area of wild magic beneath Castle Crag. Somehow, he managed to steal a part of Mystra's divine essence from the zone.
Tempus, upon learning of Malyks actions descended into the Underdark and destroyed the lich, hoping to take some of Mystra's power for himself. Unfortunately for him it seems Ao decided to grant Talos that power instead. Though no one save the new Mystra would ever know that was the case.
Malyk had no priests per se, instead he granted a few divine spells to the wild mages who followed him. Most of his worshipers were drawn from young drow spellcasters who had been experimenting with wild magic. Though wild mages of other races did take up his cause, the vast majority were beings usually found in the Underdark. Though his cult was mysterious and mostly unknown they were believed to be working towards the destabilization of Lolth-controlled settlements.
Malyks worshipers are also thought to have gained control of the powerful artifact known as the Guardian's Tear. They tried to use it to overthrow the Conclave of Sshamath who had branded wild mages as terrorists.
Lolth obviously resented drow mages being drawn away from her service and, knowing that many harboured rebellious thoughts anyway, decided to grant those living in Menzoberranzan new and powerful arcane spells in an attempt to keep them loyal.
Malyk could call upon the services of numerous wizshades, incorporeal creatures created when a spellcaster is caught in their own magical backlash.