Evil Side (SPOILERS) feature

How to join evil side:

Source: Forums.taleworlds.com

Choices at character creation are not related to joining the dark side AFAIK.

1. Before you go in the town where"Borcha" is jailed (Rivacheg), you 'll meet a guard that demands a fee. Choose answer 1: "Good! I'll pay with my fist"

2. When you first meet the "Dark Pilgrims" where "Ymira" asks for help. Choose answer 2: "Talking about sacrifice, give me your horse and all your money"

3. After speaking with "Count Arrasies" about the "Dark Force". Mr. Guy talks with you, choose answer 2: "I'll make my decision according to the circumstances"

4. When you go to talk to all kings, had one king asked you to set peace with another king. Choose answer 2: "I'm afraid it's too difficult to persuade only with a letter"

and when go to talk to king. choose answer 1: "Whatever you think. I just brought a message"

5. When you finish the fight with "Baheshtur", "Borcha" will talk with you. Choose answer 2: "I won't let that happen"

6. When you first meet "Fenrir" at Azgad. Choose answer 3: "No, they should be given a chance to repent"

7. When you meet "Melva". Choose: [INT] "To catch bandits first catch the chief. It makes you show up"

8. When you first meet "Garneev". Choose answer 1: "For glory and wealth"

9. When you win the national tournament and go to talk with the king. King'll ask you about how to deal with the dark forces. Choose answer 2: "Yes, only the powerful monarch can be able to end the time of misfortune"

10. When you meet "Garneev" again. Choose answer 1: "Yes, that mighty power"

11. When you return from the island, you'll meet "Fenrir". Choose answer 2: "Only the final victor can be the hero"

Then it will show two final choices, YES or NO...

Summary:

Source: Forums.taleworlds.com

At some point, before the siege of Zendar, you get to choose sides. One of the bad guys (Fenrir) will come to talk to you. Then you hand over your resumé, and if he doesn't like it, your only option is to confront him and run to defend Zendar.

Just after choosing evil side, each of your companions will leave your party for different reasons, you will interact with each before they leave, they will become lords/lady of the realm against you.

Jormungand will come meet you with a good number of dark armies, you will command them and must siege Zendar (you can get them in your party). Siege Zendar, and fight for the dark forces!

After the battle go talk to the dark quartermaster. He allows you to buy evil side weapons, evil soldiers (starting as pilgrims and then upgrades) and some other little things (including banner choice). Hint: before leaving Zendar you might want to use the mirror to look for that hidden thing

Note that all the good Zendar NPCs are dead now, no more tavern (door locked), or other places, you can only interact in the main Zendar square.

Leaving Zendar will bring you in a short story of what happens in the realm, and how the chaos is coming.

Your last task will be to finish king Ragnar, but Rolf will come and help the king escape, Rolf will challenge you to a hard duel, kill him and get the 4th and last gem.

This is the end of the first chapter. Continue as you want.
Note: before the end of 1st chapter, Hel and Fenrir will join your party, they are awesome and useful warriors.