I'm developing a 2D stealth game but I cannot figure out how to create something like a torch (flashlight) that the enemies hold and look around with.Any help? Also, if relevant, I'm using MonoGame.
If I have understood your question correctly, what you want to have is e.g. a "dark map", where you can have a circular area whoch is brighter from the torch.
Now, since I don't really know Monogame, you could fill your screen with a half transparent, dark color over your normal map. Then, you probably could draw a sprite over it with a gradient from the aforementioned color to a fully transparent color. You'd have to use subtractive blending to clear the area from the dark color.
I mean something like that:
This was created using an image editor, I took the image, filled it with a 65% yellow-brownish color, then made a selection around the torch source, deleted it and added a gradient from fully transparent to the previous color. For your game you could also create this image in a pixel editor and then just draw it - either clearing the area below, or using subtractive blending if thats possible in monogame.
For example in JavaFX, you can clear a rectangle of a drawing canvas with the method clearRect(), for reference.