The Catch Skull is found on the level The Storm. It is a fairly easy to find skull but it takes a bit of skill. Once you come out into the area with the two anti-air Wraiths, immediately pass the first one by and go over towards the second. There is a silo set up there that has a small valve on top where the skull rests. You need to find a safe haven from Wraith and Ghost fire that will allow you to set up a quick grenade jump to get on top. If not, have a partner get beneath you and do a modified butterfly jump to get on top. The skull is right out in the open, just a bit harder to get. Grabbing it nets you a Catch achievement for 10 G and its abilities in Campaign.
This skull sort of suits its name. Catch makes it so more grenades are thrown, both by enemies and by allies. It also makes it so that each enemy killed can and will drop two grenades each. This make campaign a lot more fun when teamed with the Cowbell Skull and also made an appearance in Halo 2. It did the same exact thing and could be found on Metropolis.
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