In the Frostwolf Keep (Horde HQ in AV), there is a cave with a number of shamans inside. The head shaman has a repeatable quest in which you turn in Blood gained from killing NPCs and Alliance players.

Once a certain amount of blood has been turned in, the shamans travel to a ledge overlooking the Field of Strife (middle ground in AV) and begin to summon Lok'ohlar the Ice Lord, a huge, hulking boss. A significant number of players need to help with the summoning, however.

It is better to wait for a certain time and then everyone with blood travel back and turn it all in at once.

In patch 2.0.1 you get rep with Silvermoon city for handing in the blood. You can't see it in your combat log, but you actually get a bit.

This is how it works. In the Frostwolf village is a small cave just to the left of the blacksmith. The primalist there accepts turnins of Stormpike Soldier's Blood, and once she gets enough she will begin walking towards the middle of the map. She's a level 60+ elite and has an entourage, so she CAN handle herself, but it's important to stay with her and keep her healed, because the Alliance wants her dead. Anyway, once she gets to the middle of the map, she will begin a summoning ritual. Once she and her assistants complete the ritual, a swirly pattern will appear at her feet. Now this is the important part: Ten Horde players must be there when this happens to right-click the swirly thing on the ground and stand still while the ritual completes. Think of it liked a warlock summon, where people have to right click the portal. This works the same way.

If the primalist makes it to her spot and summons, and if ten Horde players assist her in summoning, she will summon Lok, who basically looks like a pale blue earth elemental. He will go to the center of the map and chill (pun intended) for ten minutes. During that time, he will attack any Alliance that get too close. He's VERY strong, but be careful that you don't let the Alliance kite him away. Each time he defeats an Alliance player, he will grow in size and power. After the allotted ten minutes, he will begin going on a rampage and destroying everything in his path. The Alliance will attempt to kite him, so it's very important that warriors, druids, mages.. anyone with an intercept stays with him. Lok is VERY powerful but he's rather stupid. As soon as one person has aggro from him, he will stay on that person until they're dead no matter where they take him. So, it's important to keep the Alliance from defeating him or letting him get pulled to some random corner of the map.

In addition to the Frostwolf and Orgrimmar rep, you get +19 rep to each of the other horde cities.(+2 if you only turn in 1 Blood)
Does not show in the combat log.

This is actually giving me 75 rep with Orgrimmar (and 18.75 rep with the other horde capitals) when handing in now.
Not having to click through the quest givers gossip options is great as well.. They should add that for other rep turn-ins too.

Confirmed, 150 blood required to summon.