This is King's Tree, an Antarctic Cottonwood.
It's on the south wall of the courtyard of the Kathlin Rory Carrick Castle. It's called "King's Tree" because it's Conor Kieran's favorite. Before returning with the Armada to Aquanus, he spent many hours reading in its shade.
Antarctic Cottonwood has been indispensable to the Saeire Insu. Through it's partially aecxal nature, it has the ability to cloak objects that are mediàlmically attached to it, which means the Castle is invisible to all but the Saeire Insu. The wood is relatively lightweight and easily treatable. Once treated, it becomes extraordinarily durable and tougher than steel. As a result, the Saeire Insu Armada is built top to bottom with it, and Saeire Insu weaponry, including steel swords and the like, is laced with fibers from it.
The original grove was planted, grown, and harvested in Antarctica upon the orders of the Samanlainen Guardian Eleysius, hence its name.