Hook: Floor Medium Narrow (Blue Rectangle)The Altar of the Voss Mystics decoration fits into a narrow blue medium rectangle floor hook.
Hook: Floor Medium (Blue Square)The Altar of the Voss Mystics decoration fits into a blue medium square floor hook.
How to Get an Altar of the Voss Mystics Decoration
Cartel Pack Decorations
The Altar of the Voss Mystics decoration can be dropped from Eternal Command Packs, which were released on the Cartel Market and bought with Cartel Coins. Cartel Packs, including the Eternal Command Pack, include a random selection of items from the Eternal Command collection of items. You aren't guaranteed to get an Altar of the Voss Mystics decoration from a Eternal Command Pack, but it is one of the few ways to get an Altar of the Voss Mystics deco apart from buying it on the GTN from another player.
If you're looking to get a Altar of the Voss Mystics, the easiest way to get it is to head to the Galactic Trade Network (GTN) in SWTOR and buy it with credits from another player. If it's very rare and not available on the Cartel Market, you could try buying from the GTN and hope you get it.