From wowwiki:

Wickerman Guardians are level 60 Elite abomination guarding the Wickerman Festival outside Undercity during Hallow's End. They are Horde guards, and will attack approaching Alliance players trying to complete the quest Crashing the Wickerman Festival. They have a powerful melee attack, and a disease cloud arua. Because they are technically guards, they are hostile to hostile mobs. Usually a mob will not enter it's agro range, but if the Wickerman Guardian happens to cross paths with such a mob (while chasing another player), its disease cloud will affect the mob, causing the mob to attack the Guardian. Usually this lasts only a few seconds before the Guardian continues along its way chasing the player and causing the mob to turn back, but an exceptionally powerful mob that can out-threat the player the Guardian was chasing will make the Guardian stop to kill the mob.

Because they are guards, attacking a Wickerman Guardian will flag Alliance players for PvP. When a Wickerman Guardian dies, it drops a Wickerman Guardian Ember. This can be "opened" by all Alliance players (uncertain if Horde as well) for the Invocation of the Wickerman buff.

This NPC hits for 2500 per blow, but lacks a ranged attack so he can be kited. Thanthaldis Snowgleam, the Stormpike Supply Officer outside the Alterac Valley entrance can kill one of them on his own if a player is able to kite him to his location, and is possibly the closest such Alliance-friendly NPC to the Wickerman Festival. This is a great way for low level players to get the Wickerman Guardian Ember without the aid of another player. If a player eats a Hallow's End Pumpkin Treat and gets the floating ghost buff (the one that allows players to walk over water), they can run across Lordamere Lake to the Alterac Valley entrance, rather than traveling through Silverpine Forest and Hillsbrad Foothills. One should take care not to run too close to the murlocs on the lake however, as being hit while under the effect of the ghost buff will cause it to disappear, and the Wickerman Guardian may quickly catch up to a player without any method of increasing swim speed.

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Happy kiting!

I soloed this as a 70 holy pally. I got him down to half health with crusader/vengeance, and hitting with Exorcism and holy shock everytime the cooldown was up. Then, when I ran out of mana, I used judgement/seal of wisdom to get my mana back up until he went down. I also had to use LoH to keep alive, and ended the fight with about 200 health and 300 mana.

All in all, I'd say the buff wasn't worth the effort.