Sky’s Mercy is a cloudship belonging to a branch of House Arelius located in the Blackflame Empire. This flying cloud conveys Eithan Arelius, Cassias Arelius, and young Lindon and Yerin. It is first described in books 3, Blackflame.
“This is Sky's Mercy, the personal cloudship of the family's Patriarch. It serves us as a mobile base when we need to take our business outside of the usual territory.” (Blackflame ch 5)
At first, a little overwhelming, and maybe even frightening:
Lindon hesitated, but Eithan didn't. He was already striding across the cloud with full confidence, his steps pressing down as though he walked across a mattress. It's a Thousand-Mile Cloud, Lindon reassured himself, just...bigger. Big enough to carry two buildings. If he'd needed an illustration of the Arelius family’s wealth and power, this would do. Eithan held the door for him as Lindon fought the wind to enter. He stepped into a cozy sitting room, all decorated in Arelius colors. Dark blue chairs and couches were arranged into a half-circle around a fireplace of black metal. A spiral staircase led up to a second story, and a pair of tall, arched windows spilled sunlight into the whole space. Through an open doorway against the other wall, Lindon saw into a second room, this one surrounded entirely in glass that looked out over the clouds. Cassias stood in the glass room over a podium that looked like the control panel . . .(Blackflame ch 5)