What did Mercy do to Reaper?
I think it's hinted at (if not outright confirmed) in Overwatch lore that Mercy was the one who was responsible for Reaper's current condition after he was apparently killed at Overwatch HQ. Her attempts to revive him didn't go as planned, and rather than being restored as the Gabriel Reyes everyone knew, his cells started to destroy and regenerate themselves in a very rapid manner, which is what causes him to sometimes appear as a black fog. This is what allows him to assume his "wraith" form and allows him to teleport.