The Eternal Gift was the ancient secret ritual of the Blackwater Kingdom. It needed to construct a miniature Netherworld through extremely rich necromancy magic power to have the executor of the secret ritual and the remnant soul imprint on the ancestor's corpse resonate.
Spellcasters could either gather those soul imprints to strengthen their Spirit or summon the corpses to rise and become their slaves.
Of course, as the price of controlling this powerful force, the former might suffer the latent risk of ending up with schizophrenia, and the latter might lose control of the corpses because they failed to control their ancestor's will.
Back when Great Sage Yeats was compiling the Astral Laws, the Undead mages from the Rotten Moon Church in the Blackwater Province hoped to compile the Eternal Gift into the catalog of Undead magic. However, they were ultimately excluded by the first Astral Council because this spell contained too much Moon Realm worship.
