The oversoul is explained in the book of sorrows. It seems to be a manifestation of the cycles of death and in actuality prevents Crota from dying outside of his realm, because he has already died a million times before. The hive can rend space apart into other dimensional realms and embody death as a realm and state of being in and of itself. Thus, killing the hive seems to only sever the links between their realms and ours and makes it so they can no longer transcend it.