Eversion starts off as cute and cuddly. You're a heroic flower out to collect gems and Save the Princess by using the Goomba Stomp on happily smiling and running goomba expies. Then things get less cheerful as you everse into slightly darker and darker worlds, until...
And then the ending... You've saved the princess, hooray! Then... the lights flicker, and the princess... changes. And walks toward you. Slowly. As the incredibly creepy X-8 music plays. Then you hear the "scree" sound effect, you get a game over screen... And the game closes itself. See it here.
And in the good ending the same thing happens...only you change as well.
The concept of Eversion itself. You're literally turning reality inside out, and just try to imagine that concept in Real Life. Just imagine some random person finding a weak point in the universe and sending us all through worlds X-1 through X-XXX.
The backgrounds in the HD edition. X-1 has some somewhat blurred hills in the distance, with some trees. X-2 makes the hills look artificial. X-3 had the world falling apart. X-4 as a totally motionless, stagnant ocean. X-5 has a lifeless wasteland. So far, it's creepy, but not to the extreme. X-6 turns the hills into Eldritch Abominations. Bet you didn't see that coming. X-7 is the same, but even creepier. X-8 introduces grasping tentacles in multiple layers of backgrounds...
And the X-8 music?
It's a bit jarring when you first reach 2-3, although not as much as it is to reach 4-5, as above. X-2 still has happy trees and flowers and clouds. X-3 turns the clouds a smoky yellow, shrivels the flowers and removes the trees.
The same goes for first reaching 3-4, where the flowers are completely dead, the goombas are one-eyed and motionless, and in the HD version, the landscape is changed into a dull green "sunset". The total lack of activity combined with the unnerving music and the desolate landscape make for an overall Nothing Is Scarier mood. Things start rapidly going downhill from exactly this point on.
8-1. It's all shiny and happy, but at the end there is a demonic hand that really isn't expected. It can't hurt you; it's just there to make you fall over backwards in your chair one last time. It's quite terrifying.
Greyish rocks appear in the later levels, but they never appear before 5-5, for some reason. X-6 makes them look edgier. X-7 causes them to turn a dark reddish purple colour and start walking around. X-8 turns them into downright monsters. The fact remains that the harmless rocks never appear before 5-5. 4-5, just before, is when demonic hands appear. 5-6, just after, is when flowers become deadly. This is the steady descent into darkness.
The faces on the blocks. in X-1 and X-2, they are happy. In X-3 and X-4, they are neutral. X-5 makes them look freaked out, probably to amplify the terror of 4-5. In X-6, they're decayed.
Decayed? They look like they're melting. In X-7, they become spirals, as if they're being warped into the red eyes we see in X-8.
Think about it this way: Zee-Tee is chosen to go on a quest because of his everting powers. He knows that it's a bad idea to go beyond X-4, since that decay can creep into the happier worlds. As he ventures, he begins to lose his mind because he has to go to lower eversions to get though the worlds. As he loses his mind, he also loses control over the everting, loosing walls of death and blood and releasing those hands. Then, depending on version, there's 2 alternate stories from here:
In the older version, when he gets all the gems, it unlocks the super-guarded X-8, which was protected because it was so supremely evil. Zee-Tee only collected the gems because his twisted mind WANTED to unleash the horrors of X-8. He then reaches X-8 and subsequently loses all control, everting rapidly between all the layers as he tries to reach the princess. Too bad his everting insanity has distorted both him and the princess!
The new version is the same, but when he sees X-8, he has enough sanity remaining to try and escape, so he everts back to X-1, only to find that it's too late for them all, X-8 has already corrupted and taken over everything! Zee-Tee's sanity evaporates as he too succumbs to the forces of darkness. *shivers*
World 3, wherein you pass a checkpoint, and without any warning, you suddenly evert to World X-4 and a giant wall of blackness appears behind you, and if it hits you, you die. Made worse by the fact that, prior to this, most of the worlds had been, while ominous, not entirely scary. Possibly made worse by the fact that it's a checkpoint, so any time you start the game again, you're greeted by giant gaping shadow wall of death.
You get another one in World 6... made of blood. This also takes you by surprise. Made worse is the fact that there are those hands popping out of everywhere.
Many of the fanarts made for Eversion are pretty damn creepy too in the spirit of the game, like the one on deviantART that shows an animation of a tiger-like character named "Sly" everting. His eversion includes him closing his eyes and his forehead splitting open to reveal a replacement eye.
The above picture is another fine example of a brief sum of X-1 to X-8.
In X-1 and X-2, the music is happy. in X-3 it's slightly less cheerful. In X-4, it's only partly scary. Then there's THIS. X-5 has a typically scary tune, but it's the last actual soundtrack, and X-6 is the same as X-2, but it's constantly changing speed and pitch. If you have your speakers high enough, X-7 is a Heartbeat Soundtrack. Finally, X-8 is... um... wow.
The interface gets pretty creepy and moreminimalistic as you go up the layers. From X-1 to X-5, everything seems normal. At X-6, the score counter breaks and starts showing rapidly-cycling numbers. At X-7, when the gems turn into skulls, the "GEMS" in the gem counter changes to "????", as if the game was incapable of describing what the gems have turned into. At X-8, only the gem counter remains. Finally, in World 8-X, the HUD is completely gone. Thankfully, time attack mode averts this, displaying a time counter at all times.