The Unwithering Realm book 2. With serious spoilers ahead.
As in, it opens with Illya and the rest piling out of the gate into another location of high danger, escaping that to find themselves deep undersea in Penny's world, far below where her people go, and still in peril of their lives. Illya refuses to accept that any of them will die.
It involves Illya's losing his memory, promising his father something, a proposed alliance, Illya's identity being established when he meets his brother and his telling things his brother never knew, the songs and magic and lures of mermaids, and more.