He is a "secret" boss in Tree of life, meaning he isn't marked on the map. I don't think there are any requirements to actually fight this boss other then to go to the tree of life, but maybe there is? If there is a requirement I would say you have to do the mossback quest series to gain access to it. Although like I said I don't think there is a requirement.
YouKai is in the Tree of Life Instance. There are two "hidden bosses" in this area. YouKai is the one located at the bottom right corner while the other is the furthest house away in the instance in the top far left. I am not sure it's called "...the great" though. I don't remember seeing that in his name and I've fought him alot. But this is the only YouKai I know of