Netherlord's Brimstone Wrathsteed

The smell isn't particularly pleasant, and it REALLY stays with you.

Riding Requirements:

This mount is available to all eligible characters on your account.
Level 10
Apprentice Riding (ground)
Expert Riding (flying)
Warlocks only

Source:

Calydus in Dreadscar Rift.
Cost: 1000

Notes:

This red-orange wrathsteed may be purchased from the Warlock Order Hall in Legion after completing the scenario to obtain the Netherlord's Chaotic Wrathsteed. See that mount for more information.

Note that you originally had to level one of your warlock artifacts before you could buy this mount, but that requirement was removed in Battle for Azeroth.