During the old world, an aged magician and her two young proteges march into the depths of the underworld to make a deal with Death using the ultimate bargaining chip, an immortal man.

When Death offers to a grief-stricken witch the life of her infant son in exchange for the life of her former lover, she must capture and kill a powerful immortal who cheats death by devouring the life force of children.

e.g.
"During the old world era, an aged magician march into the Underworld to ask Death to give her back her long lost infant, finally accepting to put in play her ultimate chip: give it up her (husband/father) who devoured him for becoming immortal."

In this version, it is assumed that she is trying to negotiate with Death for a long time.

Maybe the writing could be better because English is not my native language.

Best luck.

When she is given by Death a chance to rescue her son from hell, a powerful sorcerer must capture and kill her former lover, an immortal who cheats death by devouring children's soul.