He doesn't mind not knowing the middle man. It is probably for the best. Although amusing since he would be very surprised. Istredd does know Geralt cares for Yennefer and Cirilla, but he doesn't typically seem like he bonds with other people easily. Then again, Lucifer seemed that way too.

I suppose my own first step would be to see if there is a spell here. Banishing probably wouldn't be a good option since that is for living creatures, not a magical mark. They could try to burn it off his skin, dig in under it and peel the flesh away. That is unlikely to work if this is more than just a mark on his arm. It is probably through his entire arm to the bone. He wants to help, if only to keep protecting Lucifer from the future. You don't know how to remove it other than pass it on to someone else?