Aion Stuff

The NPC is in Theobomos Lab (bottom left corner) and you need to be at least level 46 to enter. There are multiple locked doors before you can reach the NPC’s location, and you get the keys from the (level 49-50) elite monsters inside. The NPC can spawn in 3 possible rooms, but is not always there. It has a very low chance to spawn in any of them, so this is not a very easy set to get.
(Original credit for the above info goes to Palapala)

Slain Archon Cloth

Slain Archon Leather -> Better Picture: Click here

Slain Archon Chain

Slain Archon Plate -> Better Picture: Click here