[..... wow, Sebastian, that is some serious lack of self-preservation you have. Granted, there is some slight intrigue in meeting a demon from another world, someone who plays by somewhat different rules -- as evidenced by the seal.
Cas takes a moment to look at it, frowning down at his hand.]
That is the Seal of Solomon. [Which, in the Supernaturalverse, is the basis for the Devil's trap.] You're... being controlled.
[Or have allowed someone to make the symbol of control on you. So either someone has outwitted you and now controls a demon, or it's a two-way street and a mark of a deal.]