Class incompatibility - Aetolia Online Help

8.3.2 Class incompatibility

Each class, with the exception of a small number of neutral classes,
are tethered to either the Spirit or Shadow realm. One can not hold active
classes from both realms, but may hold any neutral class in conjunction
with either tethering.

The list of classes and their tethering is as follows:

Ascendril       Lycanthrope        Praenomen
Shaman          Monk               Cabalist
Luminary        Syssin             Indorani
Templar                            Carnifex
Sentinel                           Sciomancer
Zealot                             Teradrim

City organizations are also considered tethered, and an individual may  
not join a city if he or she has classes which are tethered to the      
opposing realm.                                                         

Currently, the cities of Bloodloch and Spinesreach are considered to be 
associated with the Shadow realm, while Enorian and Duiran are          
associated with the Spirit realm.

It is possible to hold classes that are not aligned with your current tether. These classes will be inactive and unable to be switched to, though it is still possible for you to become apprenticed into an inactive class. Your tether will only ever change when joining a city.