The Arms of the Bastard Prince is a set of very Ferelden-y armor that will spawn after Alistair spills the beans about his dad. The armor is a green-and-black-themed recolouring of the “Arms of Mac Tir” set, originally from the DA2 Rogue Item Pack II and ported to DAO by the most excellent Entopie, paired with a “barbarian” style helm, culled from the base game and similarly coloured. The armor will only appear for Alistair, and will only appear if he is wearing the Ferelden gear.

Known issues:
The armor does not spawn immediately after the plot-trigger conversation. It will, however, spawn on the first reload after that event.
If Alistair is not included in the party on the initial foray to Redcliffe and his background is not discussed at that point, the armor will probably never spawn, even after the truth is ultimately revealed.