Oddball Carrier Kill
From Halopedia, the Halo wiki
The Oddball Carrier Kill medal is awarded in Halo: Reach and Halo 4 for killing an opponent who is carrying the Oddball. In Reach it is represented by an octagon with a targeted Oddball skull. In Halo 4 it is represented by the Oddball symbol with an 'X' on the forehead on a grey triangle.
The medal is accompanied by the message: "You stopped (player)".