The Armour of Midnight is a suit of heavy armour. In addition Malekith has a 2 + Ward save against any non-magical attacks (ie, not spells or magic items). Also, he will never suffer more than one wound from a single hit, so war machines, magic items and other attacks that cause multiple wounds only ever cause one wound at a time. The Armour of Midnight does not prevent Malekith from casting spells.