Spell:Avatar Form

Birthright spell

The priest transforms into the avatar of their god. The spell is only usable by those with a great (or true) bloodline of the appropriate type as their patron god. Source: BoP.

For most spells the caster summons mebhaighl of the land, shapes it with their mind and carefully releases it in the desired form. With this spell the priest simply opens a connection to their deity's realm and lets the mebhaighl pour into their body and soul. For the duration the priest becomes an avatar of their deity. The immensely draining spell drains 1-2 bloodline points of bloodline strength from the caster per day. A caster drained to zero bloodline is utterly consumed.

For the duration of the spell the priest is effectively removed from play and replaced by the Avatar. The Avatar acts as it wills - typically given that the spell is only used at times of great danger to the faith, by one of true piety, the Avatar's actions are likely to 'answer the prayers' of the casting priest.

This spell is likely to either leave the priest dead (their spirit follows the avatar to its home plane) or considered a living saint by their people. As the priest is taken over by the will of the avatar they should ensure that any plans they might have are set in motion before casting the spell.
Quite what form the avatar takes is at its whim. Sometimes they might simply possess the body of the priest, adding their own strength and abilities to it. Sometimes they might shape the body into a favored form, sometimes they might take on truly unearthly attributes.
The sending of avatars by the gods is rare, and rarely passes unnoticed by the other gods. Use of this spell against other folk of godly mien is likely to invoke the interest of their patron diety...

In AD&D this is a quest spell of the sphere 'All'.