Wait, where is Otto Hightower?

In a montage at the end of the episode, we see Otto Hightower (Rhys Ifans) behind bars somewhere. The last we saw of him, he was leaving King’s Landing after being sacked as Hand of the King. The last we heard of him, Aemond told Larys to go fetch him, and Alicent was worried he wasn’t responding to her letters. Welp, that’s because he was captured. By who? It’s unclear, but it’s probably someone from Team Black.