After you finish the quest from Mikul, you go back in the crypts again and you will see that the rocks that block the way is removed and you can go and find the creature. If you have it you must finish it because he will not spawn. Nadir - you find him near the old mill, make sure is night when you attack and you don't have the quest Strangers in the Night. Royal huntsman- you can find him past the first two abandoned huts west of the Outskirts Inn to give the heads. Reminder : You can't have more then one head on yourself, so make sure you give it to the quest giver before you go after the other one. You will have 2 monsters for each chapter, make sure to give the heads on the specific chapter.
You get this sword from Royal huntsman, after you bring him 10 trophy monster heads.