While browsing a compilation of artifacts in Anima's Prometheum Exxet I discovered Dunkelschwert, a demonic sword that transform its bearer's essence into a quasi-demonic one. What is a quasi demonic essence and what are its effects on a character?
Well, my best guess is that the people affected by the item could follow the things described in "Anima those who walked amongst us". In particular on page 38, where it mentions Demonic souls. A way to interpret it might be treating the person as if they were a fourth or fifth hierarchy demon.
The book also mentions a special way to handle Demonic familiars, could treat the weapon as if it were a demonic familiar. Strictly speaking, the sword would definitely have some RP implications.