Purgatory’s Key
(Star Trek: Legacies #3)
by Dayton Ward (2016)

Romulan agents have succeeded in stealing the device and using it to banish Ambassador Sarek and Councillor Gorkon to an unknown realm in the midst of their groundbreaking Federation-Klingon peace negotiations.

With time running out as interstellar war looms in one universe—and alien forces marshal in another.

Dayton Ward brings the trilogy to a conclusion. He is saddled with a lot of plot to wrap up. And adding a strange non-physical realm doesn’t help. But it does conclude with the main characters intact and ready to embark on the journeys laid out in other media to follow.