The definition of a "rare" in uo is pretty general nowadays. the majority of items in these forums are em items, which were at a time not considered to meet the definition of "rare" used by some.
the armor only spawned for a relatively brief period of time and no longer spawns in game, so it meets the generally accepted classification of a "rare". It was from the 2008 "Shadowlord Invasion" or "War of Shadows." Basically, this was to encourage faction play right when factions were revamped. If a chaos faction stole a town sigil that town would be invaded. I forget if the sigil had to be purified first for the invasion to occur. So if Britain's sigil was stolen, Brit Trammel side would be invaded.
I stole alot of sigils during that time to get the invasions to occur, and then kill the invaders. the normal troops had studded armor, the higher ups had ringmail, and the generals had plate as well as the invasion spellbooks (max either mana regen 3, or 10 dci) After enough troops were killed the Crimson dragon would spawn, which when killed would explode into specially graphiced gems. I forget if the dragon had any type of special loot. I think it was just peerless loot, like chance of crimsons cinctures and whatnot.
This was before natural dyes so the hued armor was kinda cool, and the spellbooks were very sought after. The Blood-red plate used to sell well, and still sells as its a different hue than any in game. great to dress vendors in.