The Greater Runestone is a special building that is only obtainable by collecting fragments of the Greater Runestone Selection Kit, given as a time reward for finishing a Viking settlement within a certain amount of time.
The Greater Runestone Selection Kit provides a choice between a new level 1 Greater Runestone building or a Greater Runestone Upgrade Kit.
The Greater Runestone produces coins and forge points in 24 hours and provides an attack boost on the 4th and 5th level of the building. When it is motivated, its coins production is doubled and all special productions are enabled. The production is also affected by the town's happiness and by boosts of certain Great Buildings and it gains additional boosts after being upgraded. The building can be plundered if it isn't motivated.
The Greater Runestone can be upgraded by Greater Runestone Upgrade Kit item. To upgrade the Greater Runestone, use the Greater Runestone Upgrade item on the Greater Runestone that is already situated in your city. Its upgraded version will then be placed directly on the same spot in your city and the production timer will not be reset. Note that the upgraded building will be from your current era, even if the old one was from one of the earlier ages.