Place apples, sugar, cinnamon, nutmeg, allspice, and lemon juice in a slow cooker.
Cook on low for 8-10 hours, or until apples are very soft and easily mashed.
Use an immersion blender to puree the apple mixture until smooth. Alternatively, transfer the mixture to a regular blender and blend until smooth.
Cook on low, uncovered, for an additional 2-4 hours, or until the apple butter has thickened to your desired consistency, stirring occasionally to prevent sticking.
Let cool completely before storing in airtight containers in the refrigerator.
Notes
Apple butter can be stored in the refrigerator for up to 2 weeks. For longer storage, can or freeze the apple butter.