The Tower of Soth is one of the Burrowers that was trapped by the Spell of Preservation.
I personally used the idea that the architecture was a mixture of Egyptian (i.e. Nithian) and organic style. It was twisted but recognizable. Until the party reached the inner sanctums of the place, it was a normal building with details that were just wrong. Like the room seemed to have a recognizable 'pulse' just out of conscious hearing. Or that a hallway had a capillary effect, organically moving just a bit... enough to feel 'wrong' but not enough to scream about. As they got nearer the 'core' the level of wrong increased until the party was walking through the circulatory system of a large wormy thing.
The Nithian hieroglyphs were also a neat way to play things up. We used a cheap book that someone got at a used bookstore as the guideline. This way I could make the players have to do some work as well. The outer rooms were filled with hieroglyphs that were recognizable and meaningful, speaking of warnings, ancient hidden things, shadows in the night... but as they moved deeper into the place, they discovered that the hieroglyphs no longer meant anything and were becoming just a collage of twisted images. In the final antechambers, the walls were just adorned gross and twisted imagery.
As they brought the artifacts into the Burrower's tomb, the place began to seethe with life. Like something very old and very terrible was waking up. Unluckily, the players didn't quite get the message until they had brought the Regalia into the 'heart' of the tower. The players thought they would confront a powerful Nithian mummy or Pharoah, maybe even the agent or invested mortal form of Thanatos himself. The problem was they had fully reawakened the thing and it had started to be alive again.
I explained the situation, that they were literally inside of the monster that they weren't supposed to be in... they had awakened it fully. And it really should have digested them at this point. The reason the party ended up as worm food was because a Wizard and Priest didn't get along in character... and didn't share the pieces of information that when together gave the missing pieces of the puzzle. I applauded my players for sticking to their roleplaying... shook my head and started polling for information for the next campaign.
I like a building sense of dread and fear and than a climax of heinous evil. Basically, there is a lot of foreboding along the way, corruption, twisting, and than the truly evil stuff kicks in. When dealing with things like the Burrowers or Entropic Immortals or Evil Gods, I believe that these incarnates of evil are palpably so. I think I read too much Lovecraft and CAS for it not to be that way.
Now, onto the Regalia of Evil...
The Regalia of Evil is a trio of Evil (notice the capital E), artifacts. They consist of a crown, orb, and rod. I modified these slightly to appear like a ankh, rod, and double crown of Nithia. The party collected them with the idea of keeping them out of the clutches of Evil minions, but in truth were delivering them to a more powerful, more subtle evil that was working at the Lighthouse. Them strong nature of these things corrupted some of the staff of the Lighthouse who were involved in the running battle through the lower levels. Then the party chased the villain. They made it to the Tower... defeated him on the doorstep then carried the artifacts into the tower.
I think the Regalia was a 2E thing... I know they were in the AD&D 2E Book of Artifacts and showed up in 3E. They are basically alignment artifacts of extreme potency and more so when combined as a set. I think the combined power was that they could make one of the Burrowers bend to the will of the wielder (if they were evil)... in truth, this was a lie propagated by an Entropic Immortal... it gave the wielder no power of them, just needed the delivery service.
There is supposed to be a Good and Neutral set as well. I just liked the concept and kinda made the powers and abilities of it my own. I'll look and see if I have that stuff in an old binder somewhere. I was making notes on my PC, but not really keeping things saved with each transition to a new computer.