Garrick's Weakness

During some events tonight, and after suffering an attack from the evil one himself, i managed to discover his weakness.
Upon his staff is a gem. This gem grants him immorality, but he must take some life from something or someone else, and then he will obtain it and thus, become immortal. Removing the staff from him would make him mortal, but this is a more difficult task than first thought; To remove the staff, he must be dead. However, since the staff grants him immortality, the staff must be removed from him before he can be killed. Obviously, this means it is impossible to do either, but then i discovered that the necklace/charm he wears upon his neck isn't simply decoration; it is the object that allows him to handle that much power. If we get the necklace from him, then we will be able to defeat him. He has also made a deal that if he obtains Seran's fire rune, he shall hand over the necklace. This sounds like an incredibly good deal, too good to be true, so we may need to form a small force in case he double-crosses us, which he most likely will. If we manage to get him to stick to his original deal, he would be signing his own defeat. Korth unite, we shall defeat Garrick!

Seems like a dangerous idea...