Creating a New Prestige Class

[NitramTM]

Hello, thanks for reading this post.
I am creating a prestige class and I am having trouble with an ability class. It says "A Corrupted Soul gains the Darkness domain at 1st level, using his prestige class level as his effective cleric level to determine the effects of its granted powers. If he already possesses the Darkness domain from another class, his Corrupted Soul levels stack with levels in that class when determining the effects of his Darkness domain powers. The Umbral agent does not receive bonus spells or domain spell slots from his Corrupted Soul levels.
If the corrupted soul has a previous divine class, he can choose it as his domain. Doing it gives the bonus spells per domain and add the nine spells to his spell list.".
I understand that my path of action is:

1. Create the ability class and make it gain access to Darkness Domaint and the powers of that domain, but not spells.
2. If the previous class had access to domains, give additional access to the Darkness domain.
3. If the same domain is chosen in both classes, stack the levels for the domain abilities from both classes, add one domain spell per level and add the nine domain spells to the class spell list.

I will apreciate any help