Replace unit stats for only one map.

The draining ability is the same as that of vampires. Otherwise, it's difficult to provide specific advice based on such a vague description... If you've done everything correctly, it should work. If it doesn't work, then something was done incorrectly. You can go to the xdb file with the stats of the Seraph (somewhere in GameMechanics) and simply replace all the characteristics with those of the Hammers of Fate. Then, rewrite the skills, copying the draining ability from vampires, and remove the spells. And that's it, it should work.