first filter.
[...] I know. And I've just told Alucard, but you can open this filter to him if you like. I'm [...] not aware of anyone else that Kell would want me to inform, besides you and Seivarden.
If that blood is his, then yes, it could be Antari magic. As far as I know, those marks are only for travel within a world; traveling between worlds doesn't require a mark, but a token. But that doesn't look like any mark I've ever seen him use. Also [...] it's hard to tell from the perspective, but is it on the floor?