Cannot seem to enter the cave, says I cannot enter the cave without the most important quest and the crystal Helmet.
I have the crystal helmet....
Have you completed
The Most Important Quest yet? If not, you have to take that first, and I believe you take it from the one of the Masters Servants.
If you completed it already, you might have to take a quest from Klavdia Kalugina (or another female NPC), that can be found in the Masters Citadel.