Trim the roast of any excess fat, and season with salt and pepper. Heat olive oil in a large skillet over medium-high heat. Place the beef in the hot skillet, and brown it quickly on all sides.
Transfer the roast to a slow cooker and top it with the chopped onion. Season with chile peppers, chili powder, cayenne pepper, hot pepper sauce, and garlic powder. Add enough water to cover 1/3 of the roast.
Cover, and cook on High for 6 hours, checking to make sure there is always at least a small amount of liquid in the bottom of the cooker. Reduce heat to Low, and continue cooking for 2 to 4 hours, or until meat is totally tender and falls apart.
Transfer the roast to a bowl and shred it using two forks (reserve 2 cups of cooking liquid, if desired). Serve in tacos or burritos (see Cook's Note).