On the evening of April 21st, Valve released an update for Dota 2 that addressed a bug allowing for the infinite activation of the Black Grimoire item on the hero Warlock. The developers have also re-enabled Warlock in matchmaking, making him available for players to pick again.
Warlock was temporarily removed from the active hero pool in Dota 2 matchmaking on the morning of April 21st. While the developers did not initially comment on the situation, the removal followed the discovery of a bug related to Warlock`s Black Grimoire aspect and the item of the same name. By performing certain actions, players could exploit this bug to use the artifact infinitely, leading to rapid hero leveling.
The bug affecting Warlock (and also Witch Doctor), which was the root cause of the Black Grimoire abuse, was initially discovered on April 12th. It involved the transfer of unique hero-specific items. There was also another issue associated with Black Grimoire: in patch 7.38b, the experience gained by the hero from the artifact per kill/assist was reduced from 250 to 200, but this change was not actually implemented in the game.
Finley Holt