Make revenant abilities natural, not spells

"A skeletal form wreathed in cold blue fire, standing astride this life and
the next. Echoes of the lingering dead flicker within its ghostly flames."

Nothing about their description or the overall concept suggests spellcaster.
As such, they're now natural abilities. A minor antimagic hit (it's still
good for some other Crypt / Tomb monsters), and more importantly also means
they can work under silence, as is relevant with either silent spectres
or as such the only un-impaired serious casters in Tomb.