Hook: Floor Medium Narrow (Blue Rectangle)The Altar of Skulls decoration fits into a narrow blue medium rectangle floor hook.
Hook: Floor Medium (Blue Square)The Altar of Skulls decoration fits into a blue medium square floor hook.
How to Get an Altar of Skulls Decoration
Cartel Pack Decorations
The Altar of Skulls decoration can be dropped from Seneschal's Stronghold Packs, which were released on the Cartel Market and bought with Cartel Coins. Cartel Packs, including the Seneschal's Stronghold Pack, include a random selection of items from the Seneschal's Stronghold collection of items. You aren't guaranteed to get an Altar of Skulls decoration from a Seneschal's Stronghold Pack, but it is one of the few ways to get an Altar of Skulls deco apart from buying it on the GTN from another player.
If you're looking to get a Altar of Skulls, 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.