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I recently had a magic item that changed the player’s races via a disease track. So they started by gaining another racial power (happen to be the Kolbold shifty) then as they moved along the track they lost their own racial power then became that race. This created a whole spin off adventure where the character had to find a cure. I was one of the most memorable things that happen in the arc. Also I gave the character a level 13 demonskin tattoo as an after effect of the disease (we were level 8 at the time).

I know this isn't death, but you could put them on some sort of disease track, that may have ups and downs along the way. It doesn't have to be a roll heal or endurance to see if you move one way of the other it could be skill challenge or having to achieve things.

Showing the mark grow would let the players know it is getting worse and smaller when better.

Alternatively the mark could even have the artifact rules, and is a echo of another soul from the shadow fell. It gives powers when pleases and takes them back when it is not.