Breeder Rank: A
Monster Rank: A (Or as high as you can get!!!)
Location: Promiass Ruins

I highly recommend a very strong monster for this!!!

From the start of the errantry, there is a section of the map in the north that has 3 hallways. The first and third of these hallways each have a King's Crest. Get these two items and head toward the room in the southeast. If you have the two items, you will find a very strong Xenon waiting for you. Defeat the Xenon to get the two items joined together to create the King's Proof which can be used to create a Xenon and Xenon will be unlocked.