The player lacks any HP, and thus will die as soon as their shield drops. This renders them significantly more vulnerable to Jul 'Mdama's Covenant as they can take far less punishment.
When combined with the Pacifist skull in Halo: Spartan Assault, it forms a lethal effect; the player commit suicide by simply shooting for too long. This is quite easily the most difficult combination of skulls in the game, but aptly it also provides the highest multiplier.
The Hollow Skull icon in both Spartan Assault and Spartan Strike, as seen on Halo Waypoint.