[Even without the rune circle involved, this blood puddle is not great!!! He doesn't need to be told twice to stop walking towards it because no way is he about to touch either one of those things right now.]

I would say it could be a transfer gate, but I somehow doubt that. Leaving one that could potentially be opened or accessed by others would be foolish.

[There's, like... no sign of what exactly might have caused that blood puddle, right...]