In addition to their experiments on living beings, Necromancers who massacred a large number of lifeforms to create more of the Undead would be hunted down by all forces. Although Necromancers were widely disliked, no one would attack one for no reason. After all, Necromancers were known for being weird.
They might come back a few days after being beheaded. People would be even more careful not to provoke Necromancers who transformed into Liches.
If they couldn't completely get rid of the Lich, the only thing awaiting them would be the Lich digging up their ancestral tomb and raising their ancestors into more of the Undead to fight them.
Toursse was relatively well-known amongst the Necromancers of the Odin Kingdom. This was not because there were no Necromancers stronger than him, but rather because no one was as famous as he was.
