A few weeks ago, I was involved in an Alterac Valley (AV) battleground in which Lokholar the Ice Lord was summoned. This doesn't happen very often, and I think it may be the first time I've seen it (though some of the multi-hour AVs I took part in back in the early days are a blur to me, now). Sadly, while this almost caused us to win on reinforcements, the Alliance used the cover of confusion to go kill our general while this was going on. But it was down to the wire, and a real blast.
If you want to know how to summon Lokholar in AV, bring 150 blood (looted from Alliance players and NPCs) to Primalist Thurloga back in your base. Once you turn in the quest, have 9 other players ready to assist you. Ride out with her to Galv's side of the Field of Strife and help her summon. Once he appears, he'll be fairly weak and will need you to kill Alliance players nearby. When you do this, he gets stronger and will eventually be nearly unstoppable.