Greatshields are former warriors and heroes of Elysium who wielded shields during life, and are bound to them in death. Like the other Exalted of Elysium, when they are defeated, their lingering souls will try to re-arm and respawn, and must be dispatched before they can do so.
'...The illusion of protection is sometimes indistinguishable from the real thing. After all, one's ability to survive and prevail in perilous matters is largely an act of will. Thus do the Exalted Greatshield-bearers of Elysium cling (I think rather desperately) to their worldly comforts, even in death. During my mortal days, a shield meant nothing to me; a mere hindrance to be punctured just as easily as flesh. I understand, however, that Exalted shields are made of stronger stuff, bound as they are to the souls of those who fell while holding them in battle.'