Unlike most of the other resources within the Clash group of games, Dark Elixir is the only resource in Home Village, that isn't currently in Clash Royale or Clash Quest.
Clash of Clans
Dark Elixir is a resource from Clash of Clans that is collected through Dark Elixir Drills and then stored in Dark Elixir Storage, much like Elixir which is collected through Elixir Collectors and stored in Elixir Storage. Players can find the amount of Dark Elixir they have on the top, right side of the screen, listed under Elixir. Dark Elixir is used for purchasing or upgrading hero class troops, aside from the Grand Warden as well as training and upgrading Dark Elixir Troops. Dark Elixir is unlocked after the player reaches Town Hall 7, instead of at the start of the game like other, similar resources.
As previously stated, Dark Elixir isn't currently in Clash Royale, however, Dark Elixir is mentioned in the Bowler's description in Clash Royale, as a substance being created by Minions.