/cast summon succubus
and another that goes
/cast drain soul
/say Mwahahaha! Your soul is mine!
This is amusing for a while, and serves to wind people in your group up no end. To indulge my RP side on my warrior I have macroed a couple of /y commands to use after a boss kill in dungeons and raids.
/y Rex Aegyptus sum!
(from Robert Westall's book "The Cats of Seroster"), and
/y Who's fucking next?
(from the excellent "The Steel Remains" by Robert Morgan). I am trying to think of some others.