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So, I imagine it'd have three routes like Undertale

But to make it unique, the main focus wouldn't be that you can choose a different path, but instead it would be that the story isn't always about you. ESPECIALLY in the genocide path.

So with that out of the way I'll start on the neutral path

You wake up in laboratory, strange spider-like creatures crawl around carrying boxes of supplies

These would act as the main enemy in this section

As you explore the laboratory you eventually find beldum, who was trying to create a powerful being known as a determination entity. And it seems to have worked. He would work as the tutorial boss, he is not using even a fraction of his power, and you can't kill him with your current equipment.

He gives you some slightly better gear, and sends you to fight anyone who opposes him. (Collector, inkmatas, etc)

However if you looked around in the lab before fighting beldum you would've found some papers, revealing some of the horrible things he's done. So either that gets you to leave his side, or you notice that something just seems off about him.

Anyway, you progress through the game normally after that. You kill SOME stuff but not everything. You're not that evil... At least not yet.

You eventually meet the collector. He knows you're a determination entity, and doesn't want to waste effort trying to fight you.

He forces you out of the realm of creation, you and your allies begin trying to figure out what to do next, when suddenly beldum attacks the citadel. This is the final boss of the neutral route.

Collector calls [redacted] the new user of destruction to help him fight off beldum

The main part of the fight would be you trying to hit weak spots on his bell/head, with your attacks getting a damage bonus because of collector and [redacted] helping you.

Eventually beldum falls, you win, and you get slightly scolded by collector depending on who you killed, but he then says that he could revive them with some negotiation with death.

And then credits roll.

Pacifist route

Same start as neutral but without killing anything. Beldum shows some confusion about why you refuse to fight, but then sends you to fight anyway hoping you'll snap out of it.

You befriend several characters, but eventually get to the Collector. He claims he will not be fooled by your acts of "kindness". Other characters argue with him a bit, and you're forced out of the realm of creation. But this time, JUST you. None of your allies.

At this point you're given free roam around the citadel as you find a way to progress.

But eventually the council of the citadel takes interest in you.

Guards snatch you, and you're taken to the central tower where they test several traps on you, before the actual fight begins.

You wouldn't be able to hit them due to them being out of reach, luckily this is a pacifist route. We don't need to attack. After several failed attempts to reason with them, your allies barge in mid-fight to help you.

Eventually the fight ends, and collector finally has a small amount of trust in you.

Credits roll

And at the end it says:
Thanks for not killing anybody :)

Ok time for the dark one.

Genocide route.

Same start, but you kill everything. Due to being higher level than both other routes you're able to deal FAR more damage to beldum before he throws you out of the laboratory.

You kill everything in your path and each boss is far more ruthless than the previous two routes.

Eventually you get to the Collector. But this time is different. He actually fights you this time.

You can't heal during the fight, you have no allies (because you killed everyone) and your attacks barely deal any damage.

During the fight collector is reminded of everything he just lost- no. Everything YOU took from him. All except one thing...

His defense slowly goes down the more you hit him, until he finally falls.

Before he dies, he says one final thing:

"W-Was it worth it?"

Then he collapses.

Astra walks in saying she heard loud noises and then sees the collector's deceased body. She rushed to him hoping he was just unconscious, but no. He was gone.

She looks at you with pure rage, then takes the power of creation from what's left of the collector.

Then the battle begins, this time you can heal, but so can she. And she has some other abilities too.

Throughout the fight she will break the buttons you use to attack, heal, etc until you have nothing left. At this point she sends out a shockwave which knocks you to the floor, then jumps up and attempts to stab her spear through your chest. Pressing a button at the right time let's you dodge the attack.

Her spear is now stuck in the ground

All of the buttons come back to your screen, damaged but still functional.

You can help her get her spear, in an attempt to fix everything you've done

You can kill her, finally finishing the job beldum gave you

Or you can simply heal yourself. And watch her struggle.

If you help her, she kills you without hesitation. Because why would a monster like you deserve redemption.

If you kill her, beldum enters the realm of creation and attempts to reward your efforts. But instead you kill him. In ONE hit.

If you heal, you're reset to having to choose again. Nothing happens. Maybe the game mocks you a little bit.

In the ending where you help her, a happy song plays while the credits roll over your corpse.

In the ending where you kill her, the credits roll while every boss stares at you. Attempting to make you feel the slightest bit of guilt for what you've done.

Another Genocide exclusive boss:
Overseer the All-seeing.
After killing Eggyhead and Omeletta and slaughtering every last one of the [redacted] who were not able to evacuate in time, two people stand before you. Ove and Essy. Each of them, wielding their respective weapons, challenge you to a battle. You enter a fight with them and attack them both with one strike.
Ove and Essy see their doom approaching and-

There's a large CRACK...

The Overseer stands with his weapons blocking the children while he takes the blow.

Overseer: k-kids... Dammit... L-listen to me when I *cough* tell you to stay out of this, ok? G-go with the others...

Ove and Essy reluctantly run away, and Overseer is left on the ground with his yolk pouring out...

Despite this, Overseer gets up. He's hanging on by a thread. He's got less than a single hitpoint. Despite this, he's still opposing you. He laughs at the player...

"You're gonna have to do more than strike me with that rusty blade."

Green light flashes across the whole screen and...

The fight begins.
It's a long bossfight, and it's impossible to hit Overseer for the majority of the battle. If you attack him, he'll dodge and strike you so fast you can't avoid it. His attacks are very fast and deal a good amount of damage. The only way to win is to weaken him by torturing him with taunts about what you've done and what you're going to do. You have to be careful with what you say though, some things won't affect him while others will only piss him off, causing him to deal more damage when he attacks. If you manage to break his will, he'll stop attacking and stop moving. You can strike him, or spare him. Sparing him just makes him angry, and he kills you with a single slash. Striking him causes him to try and block your attack with his Sword and Battle Axe, but to no avail. He falls to the ground, but doesn't try to get up.

Overseer: Well... You got me. I... I may have failed... But I won't lose hope. I know, that someone out there... Hero, villain, doesn't matter. That person...
They will strike you down.

And with that, Overseer takes his final breath, and falls to the ground as his shell breaks into hundred of pieces.

Oh and in the genocide route you can unlock The Traveler by going in a crystal cave

Nobody cares my guy, stop advertising your characters on every post

well devilish responded and i told him more about the traveler's bossfight

Maybe his ability would be that you'd have a small chance of teleporting when you get hit instead of taking damage

Oh sorry not as a playable character, as one of the hardest bossfights

In phase one, he does like asgore and destroys your mercy button (if there is one) and comments "You wont need that anyways"

In phase 2 he destroys your "Item" button (if there is one) so you cant heal
You can also talk with him in this phase, this can weaken him or make him stronger

In phase 3 he will destroy your Act button (again if there is one) and so you cant do anything but fight
The rest of the bossfight ideas are here
imgflip.com/i/7ptnpa
imgflip.com/i/7q43uo

Oh wait I thought you meant unlock him as an ally like life and her healing ability

Oh after some thought i think life should be like a shop keeper rather than a ally

Well if you DO complete the bossfight, the next run will have the traveler chilling next to the lab exit, and he will be your ally
(actually he is just with you so he is sure you dont go down a dark path again)

I like your idea but do you mind if I come up with an idea?

You can make your own, this is just a concept. It's why I didn't go too far into what the story would be other than the endings

Oke!