Hook: Wall Large (Purple Rectangle)The Banner: House Thul decoration fits into a large purple rectangle wall hook.
How to Get a Banner: House Thul Decoration
Cartel Pack Decorations
The Banner: House Thul 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 a Banner: House Thul decoration from a Seneschal's Stronghold Pack, but it is one of the few ways to get a Banner: House Thul deco apart from buying it on the GTN from another player.
If you're looking to get a Banner: House Thul, 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.