I've been waylaid by the Death Flu for a week now. In lieu of flowers, send NyQuil.
Could you give us a brief introduction to the lore behind Shadowfiends? (Especially as concerns healing priests?)
The wikis have no information on the subject, nor does wowhead. And it seems highly odd that, well. As a healer priest, I get to--every three minutes--summon a misshapen, eldritch beast of shuddering flesh from beyond the veil, throw it at my enemies, and use it to suck out their life force. (Then, once it has outlived its utility, I banish it back from whence it came. No care or feeding whatsoever: just go.)
*Is* there any shadowfiend lore?
No official lore, really. Option 1: the shadowfiend is either a construct or actual living being from a different plane -- specifically, the unknown plane that shadow magic is drawn from -- summoned in temporarily (and at great effort).
Option 2: they're just a psychic illusion like many other shadow priest spells.