Highlord's Vigilant Charger

His blank horse stare is "vigilance," actually.

Riding Requirements:

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

Source:

Crusader Lord Dalfors in Sanctum of Light.
Cost: 1000

Notes:

This purple-and-silver steed may be purchased from the Paladin Order Hall in Legion after completing the scenario to obtain the Highlord's Golden Charger. See that mount for more information.

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