From these little bits of information we can extract:
- Serana was sealed within Dimhallow firstly, than the chamber was build and gargoyles were put to guard her.
- Serana remembers the way out
- One chamber in Dimhallow has an arena with a fire pit in the center. And ofcourse a dragon wall - very rough translation :
Here lies (the) body of Zvala
Who (word needed) strength to
(word needed)
(word needed)
but not stamina to
(word needed)
many
- She is not suprised about dragons roaming the sky. - which could be an oversight or I am completely wrong on what I will write next.
- She is suprised by Cyrodillic Empire - Empire dates : Septim (2E 897- not ended) , Remanian(1E 2703- 2E 430)
or Allesian(1E 243 - 1E 498) empires. This can mean that she was put in the tomb before Remanian or Allesian empires times. Or Could be tied up to a Tamrielic war which will be presented in TES:O. My guess goes to after Remanian and before Septim. As she is suprised that there is an empire in Cyrodill and she knows the name of the province which was named after Reman the first. Maybe there will be a sidequest in TESO about Serana? Who knows.
Another evidence for this to be actually true is the strong presence of Molag Bal in this time period.
- Harkon and his family are nords, more sophisticated and not afraid of magic. And Harkon called himself "King" that leads me to believe he was an independent King in Skyrim(as there were some of the territories who were not jarldom but kingdoms in High Kingdom of Skyrim) or High Rock (when Skyrim hold lands there) . Serana comments on Solitude and Windhelm - that she read about them, and the Reach that "her father didn't want to live there". So we can rule out those as Harkon's lands. Riften would not be it becuase the ruler of the Rift was a Jarl - as we are told in the ancient dawnguard backstory. My guess would be Falkreath as it was described as a powerfull Colovian kingdom, and as I recall Harkon described himself as a warlord. Also there is an ancient vampiric castle called "Bloodlet Throne" and with Dawnguard installed, there are gargoyles there.
To sum up. Harkon in my oppinion was a Colovian warlord with Nordic ancestry. The ruler of Falkreath. And he was turned into a vampire lord at the time of Molag Bals invasion on Tamriel. After the invasion was over, he fled to Castle Volkahir (as being the worshipper of Molag Bal that could be a big problem from the population) , that's why Cuhlecain and Tiber Septim could conquer Falkreath Kingdom easly - the kingdom would go to despair and civil conflict without a ruler . When Molag's Bal invasion failed he found a prophecy about vampires and got obsessed about it. That lead Valerica to hide Serana and herself from him. So with this estimates I propose Harkon's age to about 1000 years.
Thank's for reading.