Yeah bad idea to have just shoved all of them in the same place...but now that it's done it's done.
what you should have done was introduce objects such as clothes from the known cats to the unknown cats.
but now that it's done...do the same thing...keep the demon cat in his own closed space for a while...but introduce toys and clothes of the other cats into his 'space'
that would be a good start....then let the cat out slowly and monitor reactions.
i'd say that this should all pass within a week...and even if they never become the greatest friends, they should tolerate each other.
meantime you can read a good vet response on this site to a similar scenario.