The Skull Shield is part of the Hallowed add-on to the Asgard Shield Mod. This Shield will allow you to block incoming frontal attacks just like all other shields. When a spider attacks you while blocking, it has a chance of becoming "Raged." This will make the spider attack you with 3 times the normal damage while bypassing any potion/armor buffs. This ability does not seem to work on Cave Spiders. It is unknown if it works on any other mob.