The Deathmarks are shadowy assassins who excel at turning up just when your opponent wished that they hadn't. Deathmarks are at their best when deploying from Deep Strike, and unlike other units in the game can do in your opponent's turn - they just have to wait for an enemy unit to arrive from reserve! Better still, as soon as the Deathmarks arrive, you can have them mark a single enemy unit you don't like the look of. From that point on, any Deathmark unit that shoots at that unit (or attacks it in close combat) will Wound on a roll of 2+.
Ultramarines are Pussies
Except Dark Angels, any chapter that sounds like a Death Metal Band can't be that bad.