FENRIS warheads had super hardened cases to contain the detonation longer and boost the thermonuclear yield. The hardened cases also made them resilient to small arms and explosions, so resilient in fact that they were regarded as useless without the detonation codes. FENRIS warheads are considered low-yield devices.
During the Battle of Earth, SPARTAN-II Blue Team recaptured 12 FENRIS warheads from the Covenant at the Centennial Orbital Elevator. At the top of the elevator, six armed warheads were smuggled aboard a Covenant destroyer while another destroyer, Bloodied Spirit was commandeered by Blue Team. The enemy destroyer was destroyed by a combination of internal nuclear detonations and Plasma Torpedo fire. Blue Team then made the slipspace jump to Onyx.
- Fenrir is a monstrous wolf in Norse mythology.