Heat oven to 350°F. Remove stems from arbol peppers and roast for 8–10 minutes, until fragrant and lightly toasted.
Cool peppers briefly, then blend them with ½ cup water until a smooth paste forms.
Add the pepper paste, tomato sauce, remaining water, vinegar, onion powder, cumin, chili powder, cayenne, salt, and black pepper to a saucepan.
Bring to a gentle simmer over medium heat, then reduce to low.
Simmer uncovered for 15–20 minutes, stirring occasionally, until slightly thickened.
Taste after cooling slightly. Adjust heat, salt, or vinegar as needed.
Cool completely and transfer to a clean airtight container.
Refrigerate and use within 5–7 days for best quality.