Evoxius
I figure that with how the story is going, he won't have an easy time convincing the basilisk, so I suppose the next best option is to learn Fiendfyre. Like, yeah, he can get the venom when it's dead, but that stuff is Dangerous to hold on to so he can't just destroy a horcrux immediately after he finds it. I do hope I'm just being paranoid though and he manages to succeed in taming it.