Dark Anchors are enormous interplanar machines that Molag Bal has created to merge Tamriel with his own Daedric Plane of Coldharbour. The purpose of the Anchors are to pull Tamriel into Coldharbour, thus merging the two realms together into a single, nightmarish hellscape. They will spawn pseudo-randomly across Tamriel in set locations, called Dolmen, and each one is protected by a large number of Daedric guardians that will appear continuously until the Anchor is destroyed by deactivating all 5 Anchor Pinions and killing a powerful Daedra once 4 of the 5 pinions have been deactivated.
It will be possible for higher level players to defeat the Daedric protectors and return the Anchors by themselves, however it would be recommended to battle them in parties, as they will be tough competitors. For each anchor that a character destroys, they will be rewarded with a bonus to the Fighters Guild skill line, progress towards achievements, and a chest of loot.
Rewards for destroying Dark Anchors are distributed evenly. Dark Anchors have a very similar use to Oblivion Gates, and they are summoned by Worm Cult Battlemages. The Fighters Guild will offer rewards such as passive skills and abilities.
The following is the ritual used to summon a Dark Anchor, it is recited by Worm cultists periodically:
"Let Coldharbour remake Nirn in its own image!
God of Schemes, we offer this victim to you!
Through the power of innocent blood, bring forth your anchor and let two worlds become one!
Bind this world with your chains! Draw it ever closer to your realm! 
Once established, Molag Bal yells:
"I will dominate this world, and all others in time. The strong should always command the weak." 
The Worm cultists will then either be sacrificed to Molag Bal or become alert and attack the Vestige.