On parts of the maps, these Dark Anchors appear. What do they do? When ever I go to one, there is nothing going on.
As far as I know right now it's possible that Dark Anchors simply bug out randomly.
They're actually supposed to be semirandom encounters for groups of players.
Gameplay surrounding Dark Anchors:
- Cultists surround one of the dolmens found throughout all of Tamriel.
- After a short summoning ceremonie/cutscene, a Dark Anchor will appear in the sky above the Dolmen and impact at the dolmen's position.
- Once this happened, whenever there are players nearby, random groups of Daedra will spawn and attack players.
- After a given amount of Daedra got destroyed, several small diamond shaped consturcts - anchor pinions - will appear.
- From then on you can no longer win or progress the encounter just by slaying Daedra. They'll respawn/return indefinitely.
- Instead you have to fill/power up the anchor pinions by slaying the Daedra nearby.
- Once a pinion is "finished", it has to be activated by a player.
- This will cause it to drop into the ground.
- The Dark Anchor will be destroyed once all pinions are gone.
- There are no additional rewards (like a chest) other than experience points and random drops.
Lore regarding Dark Anchors:
Molag Bal tries to pull Nirn into Coldharbour (his plane of Oblivion). To achieve this goal, his followers will summon magical constructs, so called Dark Anchors. They're similar to the Oblivion Gates Mehrunes Dagon spawns a few hundred years later during the Oblivion Crisis (The Elder Scrolls IV: Oblivion). However they're not just portals into pocket realms.
Yes, there is a reward given once they are completed, as a chest drop near the stairs of the altar.