Invincible

Invincible, or Boogeyman in Halo Infinite, is awarded in Halo 3, Halo: Reach, Halo 4, Halo: Spartan Assault, Halo: The Master Chief Collection, Halo 5: Guardians, and Halo Infinite for killing 30 opponents in a row without dying in multiplayer and Spartan Ops. In the Halo 3: ODST, Halo: Reach and The Master Chief Collection campaigns and Firefight it is awarded for killing 100 enemies in a row.

In Halo 3, it is represented by a light blue target inside a black hexagon, and is the highest spree medal.This likeness later became the Unfrigginbelievable Medal in Halo: Reach.

In Halo: Reach, it is represented by a light blue target on a blue 20-point star.

In Halo 4, Halo: The Master Chief Collection and Halo 5: Guardians, it is represented by a white target on a blue ten-pointed star.

In Halo: Spartan Assault, it is awarded for killing 30 enemies in a row in co-op without being hit, and is represented by a blue shield with a Pod infector over a single inverted chevron.

Trivia

This medal is usually achieved by playing Big Team Battle and killing people instead of actually trying to get the flag, bomb, etc.
Earning this medal in Campaign or Firefight in Halo: Reach will unlock Jun-A266's helmet for the player's Xbox 360 avatar.