The terrible secret that would be kept from the rest of the players for the next several months, was as follows.
There was no "thief" who snuck into the tower and stole the mirror and crystal ball. The very thing that drove the characters on a chase into the Temple of Elemental Evil was a lie, perpetuated by Athan. It seems that Athan's curiosity always has a way of getting the better of him. In this case, during his watch overnight, he decided that he wanted to experiment with the Golden Skull and the Crystal Ball to find out what they were and how to use them. Being a supra-genius, he was certain he could handle these items without risk to himself or the party. But what Athan and Sarril didn't know (and didn't have the means to find out), is that the crystal ball was actually a Crystal Hypnosis Ball, and it was attuned to the powerful demoness, Zuggtmoy, goddess of fungi, who was trapped and bound within the temple. This magic item is one of the few methods of communication she has to reach the outside world, and she was intent on using it to its fullest.
According to the Dungeon Master's Guide:
Crystal Hypnosis Ball: This cursed item type is indistinguishable from a normal crystal ball, and it radiates magic, but not evil, if detected for. Any magic-user attempting to use it will become hypnotized, and a telepathic suggestion will be implanted in his or her mind. The user of the device will believe that the desired object was viewed, but actually he or she became partially under the influence of a powerful magic-user, lich, or even some power/being from another plane. Each further use will bring the crystal ball gazer more under the influence of the creature, either as a servant, tool, or possession object.
This was not good for the unfortunate Athan, who immediately became a thrall of Zuggtmoy. To make matters worse, he did not even attempt to resist her when he discovered who she was. He was curious, and thought that by continued engagement with her, he could find out useful information to help the party. And so, every night, starting around the 18th Day of Planting and continuing to the 24th, Athan used his time on watch to sneak outside and communicate with his mistress. By the second or third time, when he began to realize this probably wasn't a good idea, he was so completely enthralled by her suggestions that he could not refuse her requests.
The thing about suggestion spells is that intelligent beings are generally aware they are under the spell, and can find ways around the suggestions - loopholes in wording of the orders they are given. But a powerful being with the intelligence of a goddess also knows this, and Athan spent enough time with her that she was able to make her suggestions air-tight. The most sinister of all being that he was not allowed to let anyone become aware, through any means of communication whatsoever, that he used the crystal, spoke to Zuggtmoy, knows who she is, or is under her spell. Layers upon layers of suggestions were placed so deep that even Athan's supra-genius brain could not unravel them. Then, only after her control over him was absolute, did she begin to make her real demands, which were:
1. To get rid of the crystal ball and the Mirror of Mental Prowess so that the party cannot get them. Athan fulfilled this request by burying both items on the temple grounds, then intentionally creating tracks down the trapdoor up to the secret temple entrance. Knowing there was a ranger in the party that could track, this would lead the party into the temple and closer to achieving the goal #4 below.
2. To collect some special spores, which Zuggtmoy's limited powers were able to create on the grounds, and place them at the entrance to the tower. These spores grew into Black Pudding, which prevented the party from leaving, driving them back into the temple.
3. Athan is never to allow the skull to leave the temple
4. Athan must encourage the party to kill Hedrack before
5. Taking the throne from the ground level of the temple to the secret room on the third level where Zuggtmoy resides, and turn the skull over to her
Over the next several sessions, Athan executed the will of Zuggtmoy faithfully, despite the internal conflict it created. And thus the story of Athan would become a tragedy. . .