Lake Verity can be found in Glacial Chasm biomes. You can also combine the Acuity Proof (found in the Team Galactic Eterna Building after beating Jupiter) with an empty map in a regional cartography table will look like this:

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

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