Erdrick's armour is required to complete a quest in Dragon Quest IX, along with Erdrick's gauntlets, helmet, sword, boots, etc.
It can be alchemised using 1x Rusty Armour, 1x Orichalum and 9x Glass Frits. The Rusty Armour is obtained from a previous quest. Orichalum can be bought from Cap'n Max Meddlin' after the hero has given him at least 80 mini medals. Glass Frits can be found near Dourbridge.