Lake Verity can be found in Cherry Grove biomes. You can also combine the Verity Proof (found in the Team Galactic Wind Plant after defeating Mars) with an empty map in a regional cartography table will look like this:

The largest island near the middle contains the Mesprit Altar and a Pokemon Trial Spawner. Here a level 78 Hisuian Goodra will spawn. Upon defeating it, a Mesprit Plume will drop from the Trial Spawner. This trial spawner is re-doable, though I'm not sure what the cooldown is.

Upon using the Plume on the Mesprit Altar, a level 70 Mesprit will spawn. An Emotion Jewel will also come up out of the Mesprit Altar.