thanks sephi that opens up a very good way of implementing those rituals 
I am done with the numbers crunching:
Death Pact (repeatable ritual, adds population to the city it is build in):
So researching those techs will give you a higher city population cap. I decided to keep the ratio of cost to added population constant, because there will be new building (they will replace useless buildings such as the infirmary) which decrease the build time of rituals (local, i.e. only in that city).
If you want to exceed the population cap you will need to build the global version, which is not limited, but more expansive and will decreases faction standings.
I post the costs of the global version later. I assume i will finish the rituals, missing pedia entries and buildings today or tomorrow. I will also have to block the building celestial compass, since it would have a too significant impact.
Edit:
Just to clarify things. Currently one population costs 60 hammers (which is a lot less then scions for example) however keep in mind that upgrading your skeletons cost population too. For example a generic Champion costs 120 hammers. The Aristrakh pendant is the Skeleton Spearmen Company which costs 25 Hammers and 2 Population i.e. it will cost 145 hammers. Keep in mind that it is weaker then the champion (if Sephi implements the noupkeep tag it will not be a drain on you gold income however). There will be buildings (currently they replace the infirmary and the theater) that will decrease the ritual cost in the city they are build in.
The Palace will start with a bit higher production bonus then normal palaces (maybe +2 or +3 hammers instead of +1) so your capital gains the starting population a bit quicker. Keep in mind you don't need food, so your cities probably will have a higher production then cities of the same size from other civilizations. There will also be replacements to lower tier buildings that are of no use to the Aristrakh like the smokehouse for example. Those replacements will be cheap and will add a bit of gold and production, to help a city start off.

I am done with the numbers crunching:
Death Pact (repeatable ritual, adds population to the city it is build in):
Spoiler :
So researching those techs will give you a higher city population cap. I decided to keep the ratio of cost to added population constant, because there will be new building (they will replace useless buildings such as the infirmary) which decrease the build time of rituals (local, i.e. only in that city).
If you want to exceed the population cap you will need to build the global version, which is not limited, but more expansive and will decreases faction standings.
I post the costs of the global version later. I assume i will finish the rituals, missing pedia entries and buildings today or tomorrow. I will also have to block the building celestial compass, since it would have a too significant impact.
Edit:
Just to clarify things. Currently one population costs 60 hammers (which is a lot less then scions for example) however keep in mind that upgrading your skeletons cost population too. For example a generic Champion costs 120 hammers. The Aristrakh pendant is the Skeleton Spearmen Company which costs 25 Hammers and 2 Population i.e. it will cost 145 hammers. Keep in mind that it is weaker then the champion (if Sephi implements the noupkeep tag it will not be a drain on you gold income however). There will be buildings (currently they replace the infirmary and the theater) that will decrease the ritual cost in the city they are build in.
The Palace will start with a bit higher production bonus then normal palaces (maybe +2 or +3 hammers instead of +1) so your capital gains the starting population a bit quicker. Keep in mind you don't need food, so your cities probably will have a higher production then cities of the same size from other civilizations. There will also be replacements to lower tier buildings that are of no use to the Aristrakh like the smokehouse for example. Those replacements will be cheap and will add a bit of gold and production, to help a city start off.