I did this on a Dark Elf Assassin.

I'd made the Dwarf Priest hostile by converting his altar with gold, which was a big problem for a while... I was getting beaten up badly, so I started reading scrolls... One was apparently a blessed scroll of familiar summoning, because a friendly monster appeared and was killed by the Priest- who then became neutral to me again!

This isn't particularly useful unless you've made the Priest hostile... and I don't know if it works if there's any other reason for Priest hostility than converting his altar (ie: killing civilians during the Greater Demon quest).