thanatos originally started in the void; a bundle of weak consciousness and raw power that had gathered together aand formed over centuries of not being disturbed uvu. so not only is he a void god but you could say he is a piece of the void itself! fun fun.
anywho teo, a creation god (and canonically the creator of the yogsummer world- more concerning lore will go up on the yogsummer blog later) fond thanatos in the void and was thrilled!! because for once there was something there, something /living/, that he hadn't created

so after thanatos was graaddually coaxed out of the void (the only home he'd ever known) teo and him were best buddies!! so far teo has been teaching him all about this big bright new world

thanatos' role is essentially to balance teo's. since teo is a creation god, you could say that thanatos is a destruction god

thanatos is an anchor; he keeps all the realms in their place and does his best to make sure things are balanced and keep him safe/help him rest when needed
so he takes some of the strain off of teo