The Oath That Walks Three Ways

You enter a circular chamber where moonlight shines upward through a glass floor and illuminates roots twisting beneath it. Blue roses grow from the walls, blooming whenever someone speaks, while silver thorns coil around a sealed archway. At the center, four ivory letter tiles rest on a blackwood lectern beside three empty grooves. A thumb-sized fey creature with moth wings and a candle-flame body lounges atop the stone, grinning as though it already knows how each of you will answer.