Like any war, the War of Heaven and Hell left forgotten weapons, armour and corpses scattered across the battlefield. In the case of the war of celestials, however, the battlefield was the entirety of Earth, and the armour weapons and corpses defy physics, and human understanding.
Bladeheight is a massive sword plunged into the earth in what was once Oregon. Standing over 2,000 ft tall, with roughly an equal amount estimated to extend into the ground, 138 ft wide and 22.5 ft thick, it is more monument to the War of Heaven and Hell than weapon.
This monument is an unearthly metal, stronger than steel, but with chemical properties akin to silver. While it is difficult to break, it can be done, and survivors began mining it, carving out chambers to live in and working the metal into swords and armour.
The people of Bladeheight are some of the most uniformly armed and armoured people in Anno Raptus. Where most survivors cobble their armament together from scrap metal, those who live in Bladeheight make great use of their hard earned "bounty." Due to the difficulty of working the metal, only a few have more than a piece or two of armour made from it.
Mithral is the metal of the angels, their swords and armour were commonly made of it--including the sword of Bladeheight. It is a rare metal, mostly found scavenged from battlefields, though since Revelation, it has been found in deep mines and floating in pools of lava after a volcanic eruption or a crack in the mantle.
It is as strong as steel, but reasonably lighter. It also has a remarkably high melting point, and thus, baring use of technology which is very hard to find post-revelation, the fires of Hell, or the flames of the oldest dragons, it must be cold-forged.
As a weapon, it defeats damage reduction which would be overcome by silver or cold iron, hence why it is used by angels, who must often fight demons and devils at the same time.