From Halopedia, the Halo wiki
Unfriggenbelievable (Unfrigginbelievable in Halo: Reach) is a killing spree medal in Halo: Reach, Halo 4, Halo: The Master Chief Collection and Halo 5: Guardians that is awarded for killing 40 enemies in a row without dying in Multiplayer and Spartan Ops, and for killing 1000 enemies in a row without dying on Campaign or Firefight. In The Master Chief Collection it is only awarded in multiplayer (which includes Spartan Ops), and not in campaign, for which the highest killing spree medal is Invincible. In Halo: Reach it is represented by an onyx/black hexagon with a blue target in the center, in the same style as the original Halo 3 Invincible Medal. In Halo 4 and The Master Chief Collection it is represented by a red and black hexagon with a skull in the center. In Halo 5: Guardians it is represented by a blue skull on top of a purple and black hexagon.
In Halo: Spartan Assault for Xbox 360 and Xbox One, the Unfriggenbelievable medal is awarded for killing 50 enemies in a row in co-op without being hit, and is represented by a blue shield with a Pod infector over three inverted chevrons.