Vicious Kaldorei Warsaber

The blades adorning this mount's armor are designed to both inspire fear and pre-slice meals for the warsaber.

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

Notes:

To purchase this armoured warsaber you must be victorious in a great many rated arenas or battlegrounds in a single PvP season. The procedure is as follows:

You must reach the PvP rank of Combatant. 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 current season's mount (not this one).
For each subsequent time you fill your progress bar during the season you'll receive a Vicious Saddle, which you may use to purchase older Vicious mounts, including this one.
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 from 10v10 (60 points / win), 80 wins from 3v3 (30 points / win), or 240 wins from 2v2 (10 points / win).