During your post-game quest, go to an underground area at Victory Road and enter N’s Castle. N is there with Reshiram and he will challenge you to a battle. The only Pokémon he uses is Reshiram. After winning, N will bid farewell to the Pokémon he has been with 2 years ago and merge it into a Light Stone. You’ll obtain this stone and use it at the top of the Dragonspiral Tower later.<br />...<br /><a href="https://www.youtube.com/watch?v=F7hGybPB65E" target="_blank" rel="nofollow">https://www.youtube.com/watch?v=F7hGybPB65E</a>