The Stonehenges (referred as Shadow Gates in foreign servers) are the gateways to the Shadow Realm, which you can only enter through Shadow Missions. They are similar to Dungeon Altars, but rather than dropping an item on the altar, you need a Shadow Mission in your quest log to use the Stonehenge. To receive a mission, visit the Shadow Mission Board in Taillteann or Tara. You may also gain Shadow Missions through Generation quests.
Partymates must be standing on the stone floor of the altar to enter with you, if they are in accordance with the requirements. If a partymate does not fit the minimum level requirement for the mission, a message will inform the party leader that the partymate cannot enter.
Please see Shadow Missions for specific information.