The tooltip says that I get one of the abilities from the minion that is sacrificed. Is there a list somewhere that shows what abilities I gain for each minion? I sacrificed my fel pup and I couldn't find an ability of his in my spellbook that I had gained.
In your spell book there is a spell called "Command demon", which you can put on a hotkey and press to make your demon use the spells that Tryana said. Once you sacrifice your demon, the Command ability stays, so you can cast, say, Interrupt or Whiplash yourself.