Battle Gargon Silessa

Silessa fought valiantly alongside her master in the great incursion by the Light, following the battle her master set her to guard over a vault room of great value.

Riding Requirements:

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

Source:

Forgotten Chest in Repared Broken Mirrors, Revendreth.

Notes:

Silessa is only available to members of the Venthyr Covenant who have upgraded their Mirror Network to rank 3. This will take some time and a lot of Reservoir Anima.

When you reach rank 3 you'll unlock the ability to repair Broken Mirrors in Revendreth, using a Handcrafted Mirror Repair Kit sold by Simone in Sinfall. When you repair a mirror you can use it to access a hidden room with a Forgotten Chest you can loot. The chest can award various goodies, including Silessa.

Only three mirrors are available for repair at any one time, and you can repair each once per day. Typically, the mirror locations change with the weekly reset, but sometimes sooner. It also appears that the mirror locations are grouped, so if you find one you can determine where the other two are by looking them up. Refer to this comprehensive Wowhead article for a full list.