From Halopedia, the Halo wiki
Incineration is a kill medal awarded in Halo 3: ODST, and Halo 3 Multiplayer for killing an opponent that has been set on fire. The player who kills the victim need not have been the player who set the victim on fire. The medal is represented by a flaming skull on a 16-pointed star.
The medal can only be earned in Matchmaking when playing on Construct. This is because Construct is the only level to contain a fire-based weapon by default: a single Flamethrower in the center of the lowest part of the map. Firebomb Grenades do not appear in any multiplayer maps by default.