So this Picture takes place in Toms Castle and that Unicorn is standing on a Chandelier. To understand the the picture, you need to read, have fun!

I need to think of a better name but for now it will long.
Destroying of the Underworld AU is set in season 4, when they enter the Realm of magic.
In this version, Tom gets stabbed by a Unicorn instead of Marco.
But to the very start.

Star, Marco, Tom and Jackie are for the first time in the magic realm. (You remember when they are in the car and wanted to get to mewnie)

They get out of the car (even Tom) and split up. Marco, Star and Tom are able to withstand the magic for now. Stars Unicorn daughter is gone in the dark magic and they don't get to mewnie. Thats the first time a black Unicorn is attacking them. Leading to Tom getting Stabbed (Where he gets stabbed isn't important, but I made it the Stomach just where Marco got it as well)

Tom is in pain or whatever and Star tells Marco that they need to leave, because of more black Unicorns. Thats when they first leave Tom in the Magic realm.

(I just noticed that I'm writing the wrong story to the picture I previously put in xD)

After a Time, in which Star and Marco think about how to save Tom and stop the magic, they re enter the Realm. I think you can know whats coming now. Tom is riding on that horse, possessed by the Magic and attacking Marco. Marco is able to Save Tom but Star isn't able to stop the magic. So they return again to the real world (earth).

Tom is still confused because he didn't gets anything and Marco is bringing him to the underworld, using the scissors. (I know they couldn't open portals but I need to right now ) Arrived there, Tom noticed that the stab wound is still there and a Black unicorn, that was inside him, (with the purple blood we saw on Marco), is appearing. That is of course not good.

Toms parents however aren't "home" at that time and Marco and Tom have to deal with this. They also don't wanna tell Star cuz she still wants to go to mewnie.

The unicorn somehow uses its magic to connect the Realm of magic with the Underworld, what leads to more black Unicorns coming. Everyone in the Underworld is slowly losing their memories, including Tom.

Though the Wound of the Unicorn makes him more affected then usual. Tom also tries to make a border in front of the magic realm portal, but isn't really succeeding.

So with a Tom that loses his memories piece by piece, Marco tries to think of a plan. Marco also repeats the sentences:

You are Tom Lucitor
Prince of the Underworld
Magic is evil
Flee from the unicorns

Tom is gonna ask ,,Whats a unicorn?" But thats part of the oneshot which I'm writing if requested.

Whatever, Marco isn't productive and decides to go to earth to get something. Leaving Tom alone in the Underworld, which isn't a good idea, even though Tom says he could handle himself. (he can't by the way)

When marco returns he figured something out. He first brings Tom back to his sences and send him to earth because the magic affects only demons in the underworld. Thats why Marco's not affected. When Tom goes to earth and a portal to the magic Realm is opening, he won't forget.

Unfortunetely Tom is half demon and half mewman, and needs to tell Marco before the portal to earth is opening. (The portal between the magic realm and earth) Because he can lose his memorie even on earth.

Tom and Marco can also conect with one another without leaving the dimensions. They have phones so they can communicate. Thats how Tom tells Marco hes half Mewman.

I didn't thought further, but thats ok.
It's already pretty long :D

If you find logic mistaken you can tell me, as well as spelling or writing mistakes. Thank you!

See you tomorrow