Plate of the Damned
The Plate of the Damned is massive in form and covered with layers of arcane etchings, yet this imposing armor rests lightly upon the wearer. It was crafted by demons, along with the Helm of Domination, and given to Ner'zhul to make him nearly invulnerable. Along with the Helm, the armor was bonded to Ner'zhul's spirit and locked within the Frozen Throne. It was possessed by Arthas Menethil at some point.
It is unknown if it is the armor seen inside of the Frozen Throne, or Arthas' death knight armor that he somehow obtained before returning to Lordaeron.