Lumos

"'Manifest your mana into a viable light source.'""-Spell page description"Lumos is a non-exclusive spell page

you can buy it for 2500 yul in the starting village

Lumos acts like a torch as it creates light around you.

The item description in the D-Dex is "A non-snap spell. This spells cast a bright light around you so you can see better in the dark."

It's a fairly necessary item, as most of the game is quite visually dark.