First of all: nice! I wonder what level the spell would be to summon it (if you don't know what I mean: the spell compendium has calling spells for the three MM inevitables). My first thought was that you would heal the con damage at the same rate you took it, but then I saw it was drain and all was well. Not to mess with how you put it, but I would also consider using it as an advance before the inevitables, if this can't get them to the trial then the ineveitables would go out and do the dirty work themselves, but that's just my style.