After the Battle of Endor, Luke make everything possible to give a worthy ending to his father. Because Vader have sacrificed himself and became a hero. We know what happens with the body and the mask of Vader. The mask is with Kylo Ren, like we saw on “The Force Awakens”. But when the Empire falls, a lot of Imperials started campaingns of desinformation to keep the Empire’s reputation alive. Across this line, there was a group called “The Acolytes of the Beyond” This group was composed by Dark Side’s fanatics with a obsessive Interest on Vader. They considered Vader better than Emperor and the Knights of Ren propably begun from here. They started search for powerful dark side artefacts and bought from a merchant a Lightsaber they believed to be from Vader. Obviously after hus death, Vader was saw like a controversial figure on the Galaxy. This is the Canon Explanation, but many people believe in that: When Vader died, Luke catches his lightsaber. And the red Kyber cristal what Luke had in Ahch-to on Episode VIII had belongs to Vader’s lightsaber.