So we are preparing my group for another long term campaign. however one of our group members seems to have an interesting but very complicated idea for a character. here is the idea

The player wants to be found covered in runes and supposedly tied to a devil/demon of sorts. He basically wants to be tied to another plane. One caveat happens when his character dies. if the character dies, he wants their soul to be tied to the nine hells. similar to Matt mercers rakshasa and how that worked. however for a fast revival (so that he could be connected to the party) a devil gives off their power to make the Resurrection process faster. his soul goes back into the body etc. each time he goes back more runes are scarring the body and he is tied to the rift in the planes. essentially the more times he dies, the more times the rift has to open up between planes making it possible for things to come from the other side. 

I told him i could try to figure it out. possibly something where he is a cleric and the more times he dies he multi classes into a warlock, converting levels he has into warlock ones each time. However, how can i twist this to make him feel empowered as a player getting what he wants (in a way) without making it seem like the campaign is all about him. i feel like if he gets all he wants, his backstory will swallow up my ideas of the overarching narrative as well as detract from others. anyone have any ideas on how to resolve this. possible homebrew would be needed i assumed.

If he hasn't picked a class yet I would recommend a Fiend Warlock, possibly tied to Asmodeus. The runes would probably be entirely cosmetic but it sounds like he is getting a free rez out of the deal so some balance would not hurt, something like anyone who can read Abyssal/Infernal reacts negatively to him, possibly hostile, and he has disadvantage on dealing with celestials. Something thematic interesting I think would be if when/if the character dies instead of him just snapping back one of the runes stretches open into a portal and an imp or some other minor fiend pulls its way through and then to rez him merges with his body/soul. So the more times he dies the more imps there are chattering away in the back of his mind, possibly resulting in one of the madness effects. 

As for how to keep this from consuming your narrative, I would recommend that you have the other players in the group give you some backstory information if they haven't, weave their family members and home locations into the background of the game. It's hard to be brooding anti-hero when the healers adopted sister, a trickster cleric gnome with a predilection for punnery and pranks shows up. It sounds like he is pushing this to the hard dark edge, so a little levity will trip him up. 

Another thing you could do is warlock (though you could do any class and make this much more rp rather than mechanic, if they pick sorceror and you have the crit role sourcebook runechild sorceror would be good). 

You could do any number of things when they die:

If they are a warlock every time they die another more difficult task is added they must complete in a set amount of time or a penalty will occur (potentially even with them dying), the more active quests they have the more diffcult new ones added are. (Basically while they cannot die they better be careful).

Another is because inifite respawn is a bit unfair on other players every time the player dies a rune turns a hellish green permenantly as soon as every rune has gone green hellfire the body is consumed and a high level (for the current pary cr) comes through and the party has to deal with it. If you want to be nice on the respawn player they can maybe have an encounter with a devil (Asmodeus if you want it to be a difficult encounter) and they must bargain for more lives.

Litterally follow rakshasa style and have it where they have to wait say until the parties next long rest meaning the party cannot use this player as their pin cushion cause if they go down while they have not come back the party is a member down. Meanwhile the solo player has to have their soul litterally survive in the nine hells from the devils there for a period of time. Say they have their equipment cause their devil friend wanted to give them a chance.