Netherlord's Brimstone Wrathsteed

This steed is sold by [npc=99178] for 1000 order hall resources, and I believe you need to have unlocked concordance of the legionfall on one of your artifacts before he'll offer it.

Calydus likes to wander on the path to your artifact forge and possily in other areas of the Dreadscar rift, except when he's on a mission, poor guy can't be everywhere at once.

A few points of information from my experience I hope will be helpful:

Lock A: level 110 - 51 points in weapon (no concordance). Did the Quest chain for the mount.
Lock B: level 110 - 55 points in weapon (concordance). Did NOT do the Quest chain for the mount.

Neither could see Calydrus when he was on mission, both could after he returned.

Lock B cannot see item on Calydrus to buy.
Lock A could see but not buy the Brimstone version of the mount (note on the item indicates: "Complete 'Power Ascended' achievement". Once achievement was completed, Lock B could buy it.

Both locks can use the Brimstone and the Chaotic versions of the mount as can my level 22 lock. (Assume the same will hold true for the Accursed version - my personal favorite - as well.)

The mount is only available to buy AFTER you complete the quest line for the green one. Otherwise the dialog box from Calydus will have nothing to say.