You can only access the temple once per playthrough, as it is only available during the quest "The Underwater Temple". If you completed the "Stolen Nymphs" quest earlier then Marina will search for you in coastal towns. She will mention Yucomb wanting your help in gaining the Diamond of Darkness, and trasnport you under the Black Sea to the temple. The temple is full on monsters and dangers and Marina is forbidden to enter, leaving you to explore it on your own.
You need to do is make your way to 4F, however, be wary of the various pitfalls in the rooms, all concealed by a thick black fog. Getting too close will result in you falling down a level (2F back to 1F, etc).
Access up to floors 3F and 4F are blocked by a group of aquans, which should be fairly easy to defeat. When you reach 4F, you will face the Guardian blocking the Fatestone. When you succeed in vanquishing the guardian and claiming the Diamond, you hear Yucomb's voice congratulate you, and warn you not to be swayed by the darkness of the Fatestone.
Return to 1F and the exit (using the pitfalls now will save a lot of time backtracking), where you will be set upon by the Minion, Strife.