Bring a medium pot of water to boil. Add guajillos and reduce heat to a simmer. Cook for about 25 minutes, or until softened.
Remove chiles from water and place in a blender along with the garlic, onion, brown sugar and one of the tomatoes. Blend on high until smooth, about 1 minute. Add the remaining tomatoes and blend on high until tomatoes are incorporated, about 30 seconds.
Season to taste with salt and pepper.
