Shadowblade's Lethal Omen

They'll never hear you coming.

Riding Requirements:

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

Source:

Zan Shivsproket in Dalaran.
Cost: 1000

Notes:

This matte-black raven may be purchased from the Rogue Order Hall in Legion after completing the scenario to obtain the Shadowblade's Murderous Omen. See that mount for more information.

Note that you originally had to level your Subtlety Rogue artifact before you could buy this mount, but that requirement was removed in Battle for Azeroth.

Introduced in:

Patch 7.2

Travel Mode:

Ground (+60% or +100% speed)
Flying (+150%, +280% or +310% speed)

Speed depends on your riding skill.

Other Mounts Using the Same Model:

Shadowblade's Baneful Omen

Shadowblade's Crimson Omen

Shadowblade's Murderous Omen

More info on Wowhead:

Shadowblade's Lethal Omen taught by Midnight Black Reins of Dark Portent