Similar to how dispersion and spirit of redemption work.I think it would be a neat thing to have as a holy priest in pvp.
"You take on the form of a spirit (something like spirit of redemption) for 6 seconds. You are immune to all attacks and you regen health and mana. You are able to move while in this form and spells cost half the mana needed and casting speed is increased by x%. This can be casted while stunned" 2 min cooldown x% base mana.