Enter Sebilis and kill "guard" mobs on the way to the Audience Chamber. The Legionaires and the Chokodai Handlers near the Kitchen and Audience Chamber can drop the key.
Once you have the key, head down to the prison area and unlock the cell with the 2 Reet prisoners.
Use the Reet Marching Orders on them and they will follow you, one at a time, to the area where the water spout is.
NEXT STEPS NEEDED
Rewards
At least 11g5s35c
Completing this quest gives +1000 faction with Synod Reet