The additional competitive multiplayer maps that came with Destiny's first expansion, The Dark Below, are now available for all players — regardless of whether or not you've purchased it.
Coinciding with today's release of House of Wolves, Bungie has made The Dark Below's competitive multiplayer maps — Pantheon, Skyshock, and The Cauldron — free for everyone in the Crucible.
Of course, you'll still need to purchase The Dark Below if you want access to all the additional PvE content, as well as House of Wolves if you want the three (or four if you're PlayStation) new Crucible maps: Black Shield, Thieves' Den, Widow's Court, and the PlayStation timed-exclusive Timekeeper.
Both expansions are included with Destiny's Expansion Pass for $34.99. Alternatively, you can purchase each expansion separately for $19.99 each.