Here is a recipe for spicy Fajitas.
You will need
Chile de arbol

(make the Fajitas HOT) You need to add 5 -15 of these.
It depends hot hot you want them.
Chile Casabel
(Used to add color and thickness to the Marinade) Use 10 - 15 depending on how thick you want the marinate as well.
Black Pepper
(Not the power but the Ball) Use 2 -3 balls
Cloves
(you only need 1 or two)
Garlic Cloves
(Depending on how much you like add 3 -5 peices)
And finally Water.
Pur that into a blender and wait 4 minutes and strain it. Soak the Fajitas for as much time as possible. You can get away with bathing them just before you stick the fajitas in the pit.
This is also good for Lean ground meat used to make Hamburgers.
I don't care too much for the premarinated fajitas unless they are from a local Meat Market.