Whenever a player sacrifices another permanent, put a +1/+1 counter on each creature you control.
11/4/2015 If a permanent is sacrificed to pay a cost of a spell or ability, Mazirek’s ability will resolve before that spell or ability. Conversely, if a permanent is sacrificed during the resolution of a spell or ability, that spell or ability will finish resolving before Mazirek’s ability is put onto the stack.
11/4/2015 Mazirek itself doesn’t allow any player to sacrifice any permanents. Its ability triggers whenever a player sacrifices a permanent because some other spell, ability, or cost instructed the player to do so.
11/4/2015 A legendary creature that dies because of the “legend rule” isn’t sacrificed.