Put everything in slow cooker. The cider or juice should not overpower apples. Cook on low for 10-12 hours.
At 10 hours, once apples are dark, immerse hand blender or transfer to a high speed blender and blend until smooth.
Put lid back on slightly ajar and let cook for the remaining 2 hours. If you're in a time crunch, place into a pot on the stove over medium-low heat and let simmer until thick.
Let cool, store in jars in the fridge for up to 1 week unless canning.