Heat oil in a skillet over medium heat; add peanuts and garlic. Stir for 1 minute.
Add the chiles and sauté for about 45 seconds. Being careful not to burn the chiles.
Transfer to a blender and the salt and water; blend until very smooth, about 2 minutes.
For a milder salsa, use less chiles de arbol or substitute some of them for Chile Japones (Japanese Chile is milder than Arbol Chile).This recipe calls for 20 arbol chiles and it comes out very spicy! So if you like things hot, keep things as is.For a milder sauce use from 10 to 12 chiles de arbol and add 1 guajillo chile to enhance the flavor and for texture.