A convex lens for magnifying details below it. Found in the Temple of Ulos.

Alone in the temple, you find a focusing lens at the base of Kynt, Satyr god of wine. It seems someone was studying this idol closely. - Eye of Ulos (RPA-ADV4)

When used in conjunction with the reflective shard, the focusing lens can produce a beam of light;

With the reflective shard, you are able to direct light through the focusing lens. It creates interesting effects when aimed at the jewels that ornament the statues of the gods. - Eye of Ulos (RPA-ADV4)

Additionally, the focusing lens can sharpen the power of the wand of truth to show past events;

The focusing lens fits nicely into the wand’s curled notch. Together they give you a vision of Taron, sitting alone in this hall on one of two thrones. The walls are bare of trophies. A messenger arrives, crestfallen, and delivers the king news that his wife, Queen Amalia, ruler of Nalos, has died in battle defending her realm from the Dragul. In a flash, you are returned to the hall of trophies. - Eye of Ulos (RPA-ADV4)

Found at the idol of the Satyr god Kynt, the focusing lens is used to more carefully inspect visible objects of Nalos for investigation;

You inspect the rubble with your focusing lens and discover a residue of blue powder.

In fact, you find nothing unusual at all in the Temple. But then, the lens only magnifies what ordinary vision can already perceive. - Eye of Ulos (RPA-ADV4)