Simplify the following text for a Grade 11 native English-speaking person.
Original: Several former slave states had already passed legislation prohibiting slavery ; however , in a few states , slavery continued to be legal , and to exist , until December 18 , 1865 , when the Thirteenth Amendment was enacted .
Simplified: The Thirteenth Amendment , making slavery illegal everywhere in the United States , was passed late in 1865 , eight months after Lincoln was assassinated .