There are different ways to build 8th city also knows as Fey city
1- Fey City Package - 1150 Gems
2- Fey City Deed - 950 Gems
3- Dark Forest Chest - 450 Gems on sale for 382 gems
4- Attacking dark forests
5- Attacking dark forest + Fey spire
5th one would be the best way if you want to save gems, I would suggest to attack dark forests first, to get Merlin's seals, you need four of them. While you are cresting for those you might get Ysbaden and Aether seal as well.
Once you have 4 Merlin's seals then you can craft Ysbaden and Aether in Fey spire. But remember dont use guarantee, cause that will cost you 999 gems in total for guarantee to craft these seals, its better to buy the deed in that case.
If you are lucky and you dont fail in crafting this is what you need.
Flax wing x28
Blood stone x60
Blood lust x5
Guinevere's Favor x14
Morgana's seal x5
Aether seal x5
Ysbaden seal x2
Merlin Seal x4 from Dark forests
Hope this gives some info
Lord Suhaib 2, 290, 313, 323, 339
The only thing you need to get a 8th city/Fey city is a Fey City Deed.