As far as a relic for Nidhogg goes, I'd suggest a root from Thor's Oak, the real life tree said to have inspired the Yggdrasil legends and destroyed by Saint Boniface. Since I don't have him, suggesting's all I can do XD

Bluebeard/Gilles De Rais also had a book which majorly boosted his power. He was a shyte Caster otherwise. XD