But the bow used to be just like any other bow? Unless his lore changed recently that I can't find, he was defending a pool of terrible corruption in Ionia during the noxian invasion. When they finally reached his city and land, instead of coming to the aide of his country men he fulfilled his duty. To not let anyone anyone near that darkness. His now dead village and family, sparked regret, remorse, and a deep hatred for Noxians. In order to get the vengeance he seeked, he took in the darkness he swore to guard. If anything, it could have been a source of the void. Or whole other terrible corrupting source that they just have failed to go into detail about.
That bow used to just be his regular bow, but the corruption turned it into what it is now. He's now a magically corrupted being, hell bent on seeking retribution for the destruction of what used to be his life.