This makes me think of the Gods in Tu'Lia who all drop items that lead up to Kirin, the God of the Gods, who is able to summon the four underling Gods. Ouryu, Vrtra, Jormungand, and Tiamat are the lesser Wyrm Gods, who lead up to Bahamut, the Wyrmking, who can summon the four underling wyrm Gods. Except this is a lot, LOT harder.
Bahamut has been given 2 BCNMs, both are uncapped and fought in a Riverne zone(not Monarch Lynn, its an island); the first fight, the "easy" one, does not have any requirements except having completed the PMs(all the way to the final Cutscene) then you can fight Bahamut but he'll be alone on this one and his attacks arent very threatning other than his Giga Flare wich is his 2hr(he does Spam Mega Flare, but can stun it easy); beating the first one gives you the key item "Whisper of the Wyrmking" and thats it.
Then you think " WTF?! What do i do with this crap??!"; well, you cant do anything with it ALONE but once you fight, and win, Ouryu's BCNM there is a chance he'll drop the Monarch Orb. When you got both the orb and the Whisper you can go back to where you fought Bahamut for the 1st time, trade the orb and allow entrance to yet another fight for yourself and 18 ppl that have the whisper in alliance; the HARD fight, the one with Ouryu, Jormungand, Vrtra, Tiamat and pets mashing at you along with Bahamut, but dont fear them(a lot), they are much weaker versions of the actual HNMs. Also, Bahamut wont summon then all at once, he'll do it Kirin style, calling them every now and then and only once per fight. From what i managed to scoop up about it, Bahamut's 2nd fight has all other stuff from the other BC but he hits much MUCH harder; with his spammed Mega Flare managing to high around 2.6k dmg and Giga Flare for pretty much instant death(i dunno how much dmg it does but its more than any HP anyone in alliance will have) along with Firaga IV, Fire V and Flare II. *ouch* I hope he can be stunned but i aint sure.
Yes, this is for the BCNM "The Wyrmking Descends". It's a 18-person, 60-minute fight vs. Bahamut. During the fight he'll summon Vrtra, Tiamat, Jormungand and Ouryu. Vrtra summons undead as normal, and Ouryu summons Ziryu and elementals as normal.