Both are hardmode weapons. Terra blade requires hallowed bars to make excalibur and broken hero swords to make true excalibur and true night's edge, as well as mythril/orichalcum for the anvil to make it, all of which are only in hardmode. Megashark requires souls of might, as well as mythril/orichalcum for the anvil to make it, which are only in hardmode.