Light is a damage type featured in Bloodstained: Ritual of the Night. Light powers are implied to have a divine origin, and thus, they are naturally harmful to demons. Heroes that at some point have to turn against the player usually have affinity with the Light element.
Dark is the polar opposite of Light, and as such, many enemies which are resistant to Darkness have a weakness to Light-based attacks and vice versa.
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Light based attacks and weapons are effective on most demons, which makes them notably effective on most enemies. There are very few enemies which aren't weak to Light damage, and even less that are resistant to it. Enemies that resist Light based damage would be most hero bosses who usually are just temporarily opposing the player, such as Alfred.