Dark Anchors or Dolmens are specific places where the forces of Molag Bal set an "anchor" point to pull Tamriel into Coldharbour. Each zone has 3 dolmens, except Cyrodiil that has 10 due to the multiple alliance areas (Starter Zones do not have Dark Anchors and so are not listed). Dark Anchors are not always active. They are the "World Events" of The Elder Scrolls Online, so periodically you will see a group of fanatics begin the incantations to summon the anchor and make it active. You can see the effects and hear the incantations from far away, and a horn will sound when the portal has opened and it's time to fight to defend Tamriel. There are several Achievements related to closing Dark Anchors, so check each zone and participate in the closing of each dolmen once.