Need advice on how to reward a player

In my campaign, players fight against cultists who want to summon their god to the material plane. The Cultists have 9 leaders and one of them, at the very beginning of the campaign, cursed one of the players with “Soul Corrosion” - this is a homebrew curse that binds the curser and the target, and gradually corrodes the target’s soul, transferring its energy to the curser, gradually weakening the target and ultimately destroying the soul and depriving him of the chance of resurrection. We discussed this with the player in advance and he agreed to this. As a result, throughout the campaign, a group of my players fought along the way and tried to slow down the curse, while the target player gradually received debuffs (small penalties that gradually added up). As a result, the players learned that the curse can be lifted by killing the one who cast it, and now (being already level 8) they go to fight with one of the leaders.
If they succeed, I would like to adequately reward the player for the hardships, risks and hardships that his character went through and he himself agreed to this kind of curse. I would like this to be some kind of permanent effect for his character or some interesting and quite useful ability, but not something that just breaks the balance. I would be grateful for any ideas, sorry for the long post