Vengeance

Sylvanas lost a part of herself the day Arthas took her life, and the darkness within her grew to fill the void.

Riding Requirements:

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

Source:

Sylvanas Windrunner in Sanctum of Domination (Mythic).

Notes:

This deathly dragonhawk drops from Sylvanas Windrunner in the Sanctum of Domination raid, on Mythic difficulty only. As with previous Mythic raid mounts, it is possible that it will be a guaranteed drop (for all party members) at the time the raid is current, but it'll become a rare drop once the next expansion is released.

Introduced in:

Patch 9.1

Travel Mode:

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

Speed depends on your riding skill.

More info on Wowhead:

Vengeance taught by Vengeance's Reins