This appears as a full set of plate mail armor, which, if tested for, will radiate magic. It has a +3 magical bonus on it, adding to the wearer's AC and Saving Throws. It may be removed easily at this stage. Testing for curses may or may not reveal the armor's true nature (GM's choice). The magic of this armor only reveals itself in true combat, at which time, it takes over. The wearer now finds themself trapped inside what is effectively animated skin, unable to remove it.
The armor takes on a life of its own and attacks any and all creatures around it at random, including fellow party members! The GM rolls for the target of the armor each round, with the armor switching from one to another at random. The armor will use any weapon at hand, or, if there is nothing, will strike with its mailed fist for 1d4+3 HP of damage.
The armor has no HP of its own: any damage done to it, is deducted from the wearer, and it will not stop attacking until either all people or creatures around it are dead (or out of sight), or the wearer is dead. The armor and wearer still get the +3 AC and Saving Throw bonus if attacked.
Controlling spells targeting the wearer (Charm, Hypnosis, Hold Person, etc.) have no affect on the armor, since it is mindless and immune to any such. A Remove Curse spell will allow the wearer to remove the armor, but will not lift the curse from it.