A Demonic throne can be built in the thronehotspot of the throne room in a player-owned house. It requires 99 Construction and 25 magic stones to build. When built, it earns the player 25,000 experience. A throne can be sat upon by the house owner or any guest within the house. Demonic thrones are often considered a status symbol, being (usually) the single most expensive item placeable in the house, costing 24,319,150.
Trivia
It is possible to make over 12 million experience per hour by making demonic thrones using a demon butler. At that rate you would reach 200 million experience in around 15 hours, although that would cost around 193 billion coins.
An Estate Agent will refer that to buy a Construction cape, you must first be able to build a Demonic Throne, making sure that one has level 99.