Tough Luck

Tough Luck is a gold skull featured in Halo 3, Halo 3: ODST, Halo: Reach, Halo 4 and Halo 5: Guardians. It serves to make enemies a lot more resilient, doing things like jumping out of the way of grenades and vehicles or going berserk a lot more often.

Halo 3, Halo 3: ODST, Halo: Reach, Halo 4 and Halo 5: Guardians
The skull in its appearances in first-person shooter Halo installments causes enemies to be a lot mor resilient. Enemies will now jump away more succesfully from incoming grenades and vehicles. Enemies like Hunters will go berserk and enemies like Jackals and Grunts will never flee.