You can only complete this quest once.
Go find Darius Moonglow at the Druid camp at Raven's Dust and tell him stones.
Find 4 Stones of different colors:
The yellow stone is given by the shadow guards
Smart list The green stone is given by the prowler mages
Blue list is given by the Minotaur Warriors, or Shaman Minotaur (to be confirmed for these)
Smart list The red stone is given by demonic trees
Return to Darius.
Gain: