Tree of Savior - Hidden Quests - Open Sesame

1. Go to the Crystal Mines
2. Complete all the quests on each of the known levels of the dungeon.
3. Go back to the entrance and interact with every purifier on each levels.
4. Reach the location of the invisible NPC in Crystal Mine Lot 2 - 1F (the entrance is well hidden)
5. Talk to it and then type the numbers you collected in local chat.
6. Once this is done the NPC will give you another quest to solve riddles. You have to kill monsters nearby.

Reward: +1 dexterity permanently