There are thirteen different Mundus Stones in Elder Scrolls Online. Each alliance territory has their own stones and in Cyrodiil one can find all of them except The Lord.
Mundus Stones gives your character a permanent but changeable buff that complement your build and gear.
It is a ”must-have” to be able to max out your character to its full potential.
To be able to use the stones you must at first have found them. The locations of each stone is found under ”Map Locations ---> Mundus Stones”
You are able to change your active stone at any given time by visiting the stone you want to activate. It is not possible to fast travel straight to a Mundus Stone.

The Stars of Tamriel are divided into thirteen constellations. Three of them are the major constellations, known as the Guardians. These are the Warrior, the Mage, and the Thief. Each of the Guardians protects its three Charges from the thirteenth constellation, the Serpent.

Here you have the three stars with every associated constellation and effects:

The Serpent. Increases Stamina regeneration by 210.

The Warrior. Increases weapon damage by 166:
- The Lady. Increases Armor by 1280.
- The Lord. Increases Max Health by 1280.
- The Steed. Increases run speed by 5% and Health regeneration by 210.

The Mage's. Increases max Magicka by 1280:
- The Atronach. Increases Magicka regeneration by 210.
- The Apprentice. Increases spell damage by 166.
- The Ritual. Increases healing effectiveness by 10%

The Thief's. Increases critical strike chance by 11%:
- The Lover. Reduces damage taken from spells by 1280.
- The Shadow. Increases critical strike damage by 12%.
- The Tower. Increases max Stamina by 1280.