The Overkill medal in Halo 2 is awarded for killing 25 opponents in a row without dying. It is available in all multiplayer gametypes and is represented by a sixteen-sided platinum star with a yellow target in the middle.
Although this medal is not featured on Bungie's website due to its rare nature, it is assumed to have a Bungie difficulty rating of ten out of ten, due to the fact that to kill twenty opponents in a row has a difficulty rating of nine.
When someone gets this medal, the announcer will shout, "un-freakin'-believable!" to the player that received it.