Working on spoilers here, but Astrid was the signature on the bottom and it was from the Assassins Guild/Dark Brotherhood. (Mentions Dark Rites and all that.) My Assassin carried a note from Astrid as well, i was attacked on the road somewhere near Markath. Flambe'd the assassin in 1 shot, was a surprise though. Im wondering if it was because i toasted a Foresworn Camp on the plains just before the mountain roads, they seemed to be at a dungeon entrance where the ritual had recently been performed... and liberally too judging by the goat heads.