RIGHT!
So Daud and Corvo both have their powers due to the mark the outsider gives them! But what about Daud's assassins? Do they all have the mark? There's quite a lot of them and the outsider says only 8 people bear his mark(from KoD beginning) + Billy Lurk(That's Daud's second in command, right?) seems to give runes away like they mean nothing! In KoD, you have the little upgrade of your "arcane band" which shares powers with assassins, but is that what gives ALL his assassins the power? How did he get it? Any help would be great!
Thanks!
(P.S. I'm only part way through Knife of Dunwall DLC, so can you not add major plot spoilers in to your answer! A "You'll find out" will suffice if the story explains this later!)

I can't remember where I got it from, but I think I recall something about part of Daud's gift being that he could share his powers with other people. So they only have their abilities through him. And I think Lurk was handing the rune to Daud because she knew it would be useful to HIM, but not to her. (I don't consider it to be a spoiler because it happens in the first few seconds of the mission )