Why was Supreme Anvil unable to kill Awakened Mordret?

In a recent chapter Mordret stated that Valor had tried to kill him but had failed. But Anvil has already shown the ability to cut concepts. Pair this with a Supreme's Will and the lineage of War God and I fail to see how Anvil could have failed to kill a mere Awakened Mordret (at the time of imprisoning him in the Chained Isles).