The outer part of the Crystal Cave is the Echo Cave, an oracular cavern inhabited by a very mean spirit, and a creature of great substance, but little form.
In the main cavern, the PCs first encounter some fey effects, including lights and glitter and an oracle willing to answer one question. But then, as they move into the cave, an echo spirit appears and attacks, bringing echoes of itself to auto-damage and control the PCs, driving them against the sharp stalagmites within the caves!
During the fight, the ochrous jelly oozes and wobbles its way up the southern tunnel, attacking anything in its path.
When the fight is over, any survivors find the eastern tunnel has the remains of some other unlucky adventurer, a scattering of currency, and a whistle.
Rewards: 300 XP, 30 gp, a magic whistle that shows a magical path.