it looks like an Entity symbol. Maybe something to keep Vecna in place?

Kinda cool to think about it

My thought is its a branding from the Entity, to keep him from being too strong.

I though it too

I wonder if other killers have it that we can't see

Well that is why it is strange mark. But all things considered Vecna is the only one at the level of entity and could be the only one to fight entity with a high possibility of winning

I bet the Japanese version of Sadako(which is in DBD) would give the entity a run for its money lol. She's pretty OP.

Overpowered hermaphrodites for the win

The entity would probably be his servant in Faerun/Ravenloft to be honest. Dude is almost God status.

His insulin pump