Vicious War Basilisk [Alliance]

Its breath is worse than its bite.

Riding Requirements:

This mount is available to all eligible characters on your account.
Level 10
Apprentice Riding
Alliance only

Source:

Necrolord Sipe in Stormwind City.
Cost: 1

Bundled Mounts:

You will also automatically learn:

Vicious War Basilisk

Notes:

This armoured basilisk was the Alliance rated PvP match reward for Battle for Azeroth Season 3. You can still obtain this old mount but it's now more complicated: you must first obtain the Vicious mount for the current season, then compete enough times to obtain a second mount within the same season. The full procedure is:

Reach the PvP rank of Combatant during the Season. You can obtain this ranking by reaching a rating of 1400 by competing in 2v2, 3v3, or 10v10 arenas and battlegrounds.
Once you've reached Combatant ranking you must win further matches in the same season, until your progress bar reaches 100% (2400 points). You'll then receive the mount.
It appears the 1400 rating is specific to the match type you earned it in. For example, if you only earned the rating in 10v10 then you cannot progress towards the mount in 2v2 (until you also earn a 1400 rating in the latter).
The number of matches required to fill your progress bar depends on the type of match. According to comments on Wowhead, by Rainewater, you require 40 wins in 10v10 (60 points / win), 80 wins in 3v3 (30 points / win), or 240 wins in 2v2 (10 points / win).
For each subsequent time you fill your progress bar you'll receive a Vicious Saddle, which you may use to purchase older Vicious mounts .