Catch (achievement)
From Halopedia, the Halo wiki
Catch
Catch is a Halo 3 Campaign Achievement. It is earned when the player finds the Catch Skull. This Achievement gives 10 Gamerpoints to the player.
It is represented by a blue circle with a skull in the middle. The skull has a plasma grenade on its forehead.
Effect: "Pull pin. Count to three. Throw." -- Every NPC carries two grenades and those that would throw them now through them faster and much more often.
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The Catch skull can be found on "The Storm" on normal or higher difficulty. To get it, you cannot die during the level, and you cannot destroy the Wraiths.