Shadow
What is your strategy for confronting the fantasy creature Shadow in Dungeons & Dragons?
More info on this creature: Source 1
Obviously shedding more light on the situation would be first and foremost. Light spell, torches, lanterns, or oil will be sufficient light sources.
I have to say I think running a campaign with shadows as an epidemic spreading across the land sounds really fun. There are so many ways a GM could use shadows in a campaign.
They have bad AC and bad HP but, the way they can drain strength and turn another into a shadow this way makes them a beautiful tool in the GM's belt.