Valve has removed the hero Warlock from the active hero pool in Dota 2 matchmaking. The developers have not yet commented on the situation. This action follows the discovery of a bug associated with the hero, which allowed players to gain experience at an extremely rapid rate.
The bug is connected to the Black Grimoire aspect and the item of the same name. By performing specific actions, players could exploit this interaction to infinitely use the artifact, resulting in incredibly fast hero leveling.
An earlier bug concerning Warlock (and also Witch Doctor) was found on April 12th. That issue involved the transfer of unique, character-specific items. This previous vulnerability, like the more recent experience-gaining bug, has not yet been resolved.