How to play as ...

From Crusaderkings

Unmodded, the game will only allow you to play Catholic and Orthodox feudal states at game start. These will also be the only states shown as available by the save files. This means, eg, that if your heir is force-vassalized and converted to a non-Christian religion, your state will not appear playable in saved games after he assumes power.

For games before CK 1.05, there is a mod that changed this. For current versions including Deus Vult, the way to play as these states is to open your saved game in a text editor. Near the top of the file, there is a list of playable states given in the format:

C001 = { gender = neutral type = example dna = "12345678901234" title = { C001 }}

To play as your realm, insert the appropriate tag line into this list. The tag must be accurate: the information needn't be and the spaces are mostly unimportant except between terms. For example, to play as the Fatimid Caliphate, the line

FATI={gender=female type=kingdom dna="12345678901234" title={FATI}}

works just fine. Listing the state as a kingdom here does not make it one: it merely means the state will be listed with the kingdoms in the start screen.

NB: The game is not designed to be played by these states.

Particularly, understand that almost always the heir of a bishopric, republic, or order will not be from the same dynasty. It's important to save your game as soon as the current ruler dies, edit that file, and then reload as the heir. Otherwise, you will simply see the GAME OVER screen on the next day or take over as another member of your dynasty in another court.