How to evolve Haunter to Gengar in Pokémon Unbound?
You evolve Haunter into Gengar with a Link Stone. Go to Crater Town and head north to KBT Expressway, and find a man standing by the wall. After talking to him, smash the walls repeatedly; you will pick up a Link Stone eventually. Or get one Link Stone on one of the houses in Fallshore, to the southeast.