I'm DMing a game using the Essentials rules/books. I see in the Fighter Knight class a power (Defender Aura) that mentions a Marked Enemy. I've also heard on the PennyArcade/PVP podcasts this marking thing as well. I can not find in the Essentials books where it discusses who can mark and when. Can someone point me to the right Essentials book/page or power? I would also be interested in hearing about Marking in general (who and when) but would really like to see the reference in the books.