Talk to the old woman by the well in Holy Village(SW) then jump in.
When you're in inside the cave, go to the square room 1st and do the puzzle. When you succeed, a door will open. Open the chest inside the room and 3 monsters will appear (level 60 or so, 2400 HP each and they regenerate). Defeat them and you'll get 2X EXP Potion and then enter the room for Elin's Key.
Now go out and and enter the 2nd room. (You need to be lvl 20+) fight more monsters (level 72 or so), and if you are successful, will get Elin to join the team.
Rewards: 1st part - x2 EXP potion and Elin's key room. 2nd part - Elin.