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Undead Phalanx...
They're the amalgamations of failed undead experiments an Archlich made in their pursuit of eternal life, more importantly, they're an attempt at the ultimate blasphemy - the creation of God and its enslavement.
Amongst the people trapped outside of the walls, Cedric has the most knowledge about the Undead; he'd been killing literal hordes of them in Purgatory after all.
The Undead Phalanx are slow but insanely durable creatures, they are resistant to all manner of physical and magical attacks - they are, after all, made by Archliches and were created to be their frontline servants, therefore they will have the highest defense amongst its army.