What happens if a druid shape-shifts into a hyena. . .

Sounds like a great idea, but I wouldn't be too overt about it.

A beastshape is a magical action that takes place over a number of hours and replaces the casters physical body with an animal one. The caster's body will still exist, probably in a pocketdimension, tied to the animal body, hence the spill-over of excess damage. The caster's body would undoubtedly recieve some of the taint, bit I don't think it would ve powerful enough to transform a PC. (While lychantropy and its derivatives seem to function on all creatures, this just works with hyenas)

Now, what dows this mean for our druid PC? Well, first of, when he next tries to beastshape into a hyena, make him a gnoll insteasd. That ought to be fun At the same time, drop hints that he now understands goblinoid languages, or whatever these gnolls speak, and that he has a perculiar longing for raw meat, even in human form.

If you then want to turn the druid slowly into a demon, that'd be fantastic XD