Well, was he actually seen to raise the dead? Or was he just hanging around when it happened? How likely are the watch to get in trouble if it turns out to be a mistaken arrest?
Marienburg wants to keep the elves happy, which they do by bending the law for them outrageously. At the least, the watch would be expected to hand the prisoner over to Elftown immediately. They are likewise expected to hand over anyone suspected of committing crimes against elves.
Have you arrested a necromancer? Or assaulted an innocent elf out on a boat trip?