Val'anyr explained

In an unusual move, Blizzard has spelled out exactly how the proc on Val'anyr, Hammer of the Ancient Kings works. There are probably one or two of you who consider it a spoiler, so I'll put it after the cut.

First, he notes that it is the proc that makes the item legendary, given that its stats are on par for epics of its ilvl. Here is precisely what that lovely proc does:

• There is a 10% chance whenever healing is done by one of your HoTs or direct heals for you to gain a buff called Blessing of Ancient Kings for 15 seconds. This has a 45 second internal cooldown. Heals that are entirely overheal – such as HoTs ticking on a full-health target – do not have a chance to proc the Blessing.
  


• The Blessing causes all of your heals (for the 15 seconds that the buff is up) to put a shield on anyone you heal absorbing 15% of the amount healed for 8 seconds.


• This shield stacks with itself, will proc from HoT ticks that occur after you gain the Blessing, and includes overhealing - if I have the Blessing up and hit someone with a 13k Greater Heal, of which 10k is overhealing, they get a shield for (13k * 0.15) = 1950 damage.
  


• There is a cap of 20,000 on the shield effect.

If all of this is confusing you, go check out the examples on Bornakk's post, which are very helpful.

Now I want this even more. It's too bad I have no interest in 25-man raiding.