Hermit M3(2), the trader will lower his prices.
Jonah M3(3), you will learn about the last this that Laiken has left on the island. You will find it in the cave M3(M3.1).
Vacca M3(5), you will learn that you can find Hermit's magic bow in the chest.
Catherine M3(12), you will get the password to the gate M3(M3.2).
Sam M3(14), the trader will lower his prices; he'll also do it if you bring him Desert Roots.
Island, you will get and additional skill point.
Elijah M3.3(3), you will learn about the secret passage M3.3(2).
Erlking M3.3(7), choose the ballad book, you'll reveal a secret room.
Sassan M3.3(9), you will get an additional stats point.
After killing the monsters summoned by Sassan M3.2(1) during The Second Coming quest M3(4), one of them will drop key K2, which opens chest S2 in the room from which you'll be going back.
Plant the explosive charge under the buried passage M3.3(5), at the end of the corridor you will find key K3 which opens chest S3.