If you side with the Assassin Lord, a new zone will open up. This will be triggered the same way the Assassin Lord quest is, starting at player level 15.

Once you've cleared this area, you can come back for access to the merchant services you would've gotten if you had saved the merchant - at a substantial markup over normal cost.

Alright, so this is two parts. A new zone, and a new class that you unlock by finishing said zone.
You should get an in-game item that explains some things.

Illusory Wood
You'll find this place a couple spaces east of Zigur. Minimum suggested level is 18.
Complete the quest inside and you'll unlock the Trickster class.

Trickster
A psionic rogue.
- Use illusory walls and decoys to distract your enemies.
- A wide variety of debuffs that confuse and distract your foes.
- Turn enemies' confusion into your own power.
- Convince your enemies that they don't see you, even as you stand in plain sight.
- Stabbing things.
The basic idea is that you throw out illusions everywhere to cover you while you stab things to death.

Feel free to offer suggestions/feedback/whatever.