Mithril

'Is everyone already in position?' Athos asked mentally. Their skeletons were lined up around the wall, breaking through the wall at the same height as Athos did. They broke just enough to reach the mithril plates and keep the wall from toppling over.

The skeletons also broke a path that connected the wall to the fortress, finding the same magical metal plates.

It was impossible to feed the large-scale spell directly, any attempt to inject mana would just make the mana circulate through the metal, but that was exactly what Athos wanted. The runes were full of energy and easily extinguished corruption, but the undead continually injected more energy.

The large-scale spells were created by over 50 mages, so they weren't in danger of exploding even if the skeletons injected corrupted mana at full strength. The runes glowed in rejection of the corrupted energy, but there were so many entry points that the energy became scattered in an attempt to protect the runes.