The wording of the Swordsage maneuver Death Mark is as follows:
When you use the strike, you channel overwhelming fiery energy into the body of your foe. In addition to dealing normal damage with your attack, you cause fire to erupt from your enemy's body in a spread.
Tome of Battle p.52
There is no mention that the melee attack has to actually hit to cause the fire damage - does that mean the enemy bursts into flame, taking 6d6 extra fire damage in any case?