Master Poisoner is a passive rogue ability learned at level 90 for those with the Assassination specialization. It grants a small boost to mastery from all sources.
Patch 3.3.3 (2010-03-23): Now correctly reduces the duration of a sting refreshed by Chimera Shot.
Patch 3.2.2 (2009-09-22): No longer increases the Deadly Poison application rate following a successful Envenom and instead now provides a 33/66/100% chance of preventing Envenom from consuming Deadly Poison.
Patch 3.1.0 (14-Apr-2009): Now also increases the bonus to apply Deadly Poison when Envenom is used by an additional 15/30/45%.