Make the NPC have the same script as of a dead branch or adjust it to your needs.
Dead branch script:
monster "this",-1,-1,"--ja--",-1,1,"";
the -1 after the "--ja--" parameter is the monster ID. If you change it to -2 it'll get a mob from the porings list and if you change it to -3 will summon just like a bloody branch. Also, if you set that to a valid mob ID it'll summon the monster with that ID.