Dimitri's brushed with a few Fears, but Slaughter's the one that's really got hooks in him -- it's the formative trauma that led to all the others (well. Corruption came free, actually, thanks Stag Beetle, but mostly).

The Tragedy (specifically, the incident where he first killed another person) left him with three long, slashed scars across his back; that seems like a strong candidate for a Mark?

("Killed another person" is headcanon, but I think it's narratively strong & supported by the text; the scars are explicit canon)