Of course it is not mandatory. You have the choice to get shadow spells or not. However you do have to go through the quest line and do the fight that makes you a shadow wizard. If you choose not to do the quest that makes you a shadow wizard then you will not move on in the game. It is part of the main quest line. But you are not required to obtain the shadow spells or use them. But why you would not want them, I don't know. It can greatly enhance your spells.
It is part of the questing in Khrysalis, but that does not mean you have to put it in your deck. However, you will always have to face shadow enemies if they're part of the quest.
spells or not. However you do have to go through the quest line and do the fight that makes you a shadow wizard. If you choose not to do the quest that makes you a shadow wizard then you will not move on in the game. It is part of the main quest line. But you are not required to obtain the shadow 
Exalted