Boil the water.
In the meantime, remove the stems and seeds from the inside of the chiles.
Place the chiles inside pot of boiling water.
Remove from heat.
Cover and let sit for 5 minutes, or until pliable.
Discard the water.
Place the rehydrated anchos, oregano, cumin, garlic, cinnamon, apple cider vinegar, salt, pepper, and water in a blender.
Blend until smooth.
Strain the sauce.
Wash and rinse the pork.
Pour the sauce over the pork.
Be sure to coat well.
Cover and let marinate in the fridge for 4 hours, overnight is best.
Place pork and all the sauce in the slow cooker.
Cook on low for 8 hours or on high for 6 hours.
Remove the meat from the slow cooker.
Let cool slightly.
Shred the pork.
Return the pork to the slow cooker and its juices.
Mix to coat in the juices.
Serve and enjoy!