Did you know that the concept of Anima, Soulbinding, Covenant and Soul contract, and soul infusion are not a New concept in WoW?
Think about Arthras and Nerzhul
Thing about the Major character being used as "power" similar to known Legendary Weapons. Think about Shaman King and souls residing in an armor or weapon as their new house or body. Similar to the Genie in the lamp or sentient weapons like BotBE or Arcane mage Lego weapon.
The weapon or armor becomes soulbound to the owner. They become one in mind and in Soul. Whatever strength or power he has he can lend it or extend the capability of those who wields or wear the armor.
Imagine Arthras as a legendary soulbind or becoming an anima infusion for armor or weapon.
Jaina, Tyrande, Varian, Shaohao, Rhonin, Medhivh, Zuljin, Garrosh, Onyxis, Nefarian, etc.