I'm thinking about making a one-shot for players of 1st level that includes a rift to the far realm, does anyone have ideas for plot or monsters (beside star spawn ik they exist) that would work in this one-shot

Aberrations for 1st level characters might be a bit few and far between, you could nab a monster from 3e d&d called "Pseudonatural", effectively you add certain abilities to normal monstrosities or beasts to reflect them being affected by the Far Realm. For a 5e version it would mean they get they get the "aberration" tag added to their creature entry, they gain the True Strike cantrip & Resistance to Lightning and Acid dmaage and then if they had 4+ Hit Dice they would also get Resistance to non-magical Bludgeoning, Slashing and Peircing damage.

You could then use these corrupted critters with one normal aberration such as a Nothic as the BBEG. The Nothic is a CR2 aberration which used to be some form of arcnae spellcaster and it has some very interesting attacks and for a low level monster it also has 120ft of True Sight which is pretty ridiculous.

EdIt: small addendum: If the PC's are 1st level, keep in mind that, depending on class and cons score, their HP will range between 6 & 17 and one critical hit might kill them outright so you might want to tweak the damage the monstes can do a little.

Thanks for the Beast & Monstrosity idea, I going to be using a Homebrew, Warlock of the Great Old One. 

where do I find the "Pseudonatural" I checked wiki fandom but it was being annoying.

try this one: https://www.realmshelps.net/monsters/templates/pseudonatural.shtml

It is 3rd edition d&d so needs a bit of tweaking to bring it upto 5e.

You can take almost anything and reflavor it to be an aberration. Just find a critter with something more interesting than a straight-up "I hit you for X damage" attack. Maybe you'll need to change its damage type. The important thing is how you describe it. Break the expected symmetries. Describe how it doesn't move normally.

Take the giant spider. Spiders are creepy enough, but we know what they are. The aberration reflavor might be a sphere so dark, that no light reflects off at all, except for the eyes. There are three of them. Sometimes they close, and reopen elsewhere on the sphere, but there are always three. It has five legs, but they don't move when it does. It just glides along the ground and up the wall, and onto the ceiling, like gravity doesn't really affect it. Instead of its bite doing piercing and save vs poison, its touch does cold and save vs necrotic. Figure out something for the webbing.

That's an example, and it may not match the aesthetic you're going for in your cosmic horrors, but you get the idea.

Just go Lovecraft. 

Start with cultists, let them explore ND fight them till level 4. Toss in some good old yuan-ti and gnolls (which are like Lovecraft's take on the ghouls, aka eaters of the dead that have dog like traits). They kill the cult leader but there's a rip in time and space and it's off to the far realm where everything eats everything. Toss in your aberrations and other unfortunate souls trapped here, and viola. Cap it off with a home brew big elder god.

This can heavily depend on what you identify as an Abberation. If going for something from the Far Realms, all you need to do is make the creatures very alienlike without touching on different planetouched creature's toes. So this would usually mean that don't make them angels or heavenly (celestials), made of crystals or forces (elementals), or be juiced up with the wilds and life energy (fey). There are more examples of course, but what you are usually left with for abberations are nonsensical biological creatures, acting as combinations of entities that do not belong together. Usually they will be fleshy and slimy and have something to do with psionics. 

A general plot idea for a oneshot of this low a level is that all the characters are from a town and a person went missing (usually a friend of all or perhaps an elder). The characters first try to investigate, and it starts out as a myster campaign for the first hour. Once they collect enough clues they find either some cave or well (whatever dark, mysterious hole you want to hide your creepy guys) that is filled with bits of flickering purple and blue lights bearing a horrid smell. They can hear the missing person's voice subtly as an echo, and their senses all tell them that the person is in there. Once the characters are in there, they will deal with some sort of abberant bug like races. You can reflavor pretty much a series of spiders and giant spiders as well as add some psionics in there to spice them up. They will have to deal with illusions and a cavern system that does not make sense with the dimensions of where they entered.

The character's goals would normally be to just save the missing person and get out, but if they are feeling more heroic, perhaps you have a psionic crystal that is keeping the rift open. To close the gate though, something of flesh and a soul must grab it from its pedestal. This forces someone to stay behind and they make a glorious sacrifice. But low and behold, you have just created the second episode of this oneshot for a few years later for the characters as they all hear the friend who stayed behind's voice as he wants to be saved.

Sorry for the jumbled narrative ideas, but I think it sounds cool.

Quote from Bob_the_fish12 >>

Just go Lovecraft. 

Start with cultists, let them explore ND fight them till level 4. Toss in some good old yuan-ti and gnolls (which are like Lovecraft's take on the ghouls, aka eaters of the dead that have dog like traits). They kill the cult leader but there's a rip in time and space and it's off to the far realm where everything eats everything. Toss in your aberrations and other unfortunate souls trapped here, and viola. Cap it off with a home brew big elder god.

 
Counterpoint: don't go Lovecraft. Those campaigns are tropey and tired.

Or go Lovecraft in a more authentic sense: portray the Far Realm as distant and horrific. The players can't go there without being utterly annihilated in body and mind. The players should focus on protecting reality from the Far Realm and fighting the servitors of the big bad that break through. In this sort of campaign, the players should never actually come face to face with the big bad. Think Sauron. If Gandalf or Aragorn actually came face to face with Sauron at full power, they wouldn't even come close to matching his power. And the players should know this. Have the big bad wiggle a finger and annihilate a city or some such.

I also have a number of homebrew aberrations on this site, though they're probably unbalanced.

Quote from Gnomarch >>

Quote from Bob_the_fish12 >>

Just go Lovecraft. 

Start with cultists, let them explore ND fight them till level 4. Toss in some good old yuan-ti and gnolls (which are like Lovecraft's take on the ghouls, aka eaters of the dead that have dog like traits). They kill the cult leader but there's a rip in time and space and it's off to the far realm where everything eats everything. Toss in your aberrations and other unfortunate souls trapped here, and viola. Cap it off with a home brew big elder god.

 
Counterpoint: don't go Lovecraft. Those campaigns are tropey and tired.

Or go Lovecraft in a more authentic sense: portray the Far Realm as distant and horrific. The players can't go there without being utterly annihilated in body and mind. The players should focus on protecting reality from the Far Realm and fighting the servitors of the big bad that break through. In this sort of campaign, the players should never actually come face to face with the big bad. Think Sauron. If Gandalf or Aragorn actually came face to face with Sauron at full power, they wouldn't even come close to matching his power. And the players should know this. Have the big bad wiggle a finger and annihilate a city or some such.

I also have a number of homebrew aberrations on this site, though they're probably unbalanced.

 Fair enough...  the biggest problem is that in Lovecraft you never meet the evil, but you also never meet the minions. Once you meet even the lowliest resident of innsmouth, it's pretty much over and you're done for. It's great for CoC but for combat and D&D, you got to show the goods. 

Truthfully I'm also thinking more of classic Conan, hence the yuan-ti. Even the cartoon has the snake people...lol. 

Travel to the far realm isn't necessary but the OP mentions it as part of his idea, and I'm just trying to give ways of leading up to that and the monsters you might see along the way.

If you keep it on terra firma the old swartzenegger Conan movies would work. Was it the second one with the elder god?

Quote from Rob76 >>

Aberrations for 1st level characters might be a bit few and far between, you could nab a monster from 3e d&d called "Pseudonatural", effectively you add certain abilities to normal monstrosities or beasts to reflect them being affected by the Far Realm. For a 5e version it would mean they get they get the "aberration" tag added to their creature entry, they gain the True Strike cantrip & Resistance to Lightning and Acid dmaage and then if they had 4+ Hit Dice they would also get Resistance to non-magical Bludgeoning, Slashing and Peircing damage.

You could then use these corrupted critters with one normal aberration such as a Nothic as the BBEG. The Nothic is a CR2 aberration which used to be some form of arcnae spellcaster and it has some very interesting attacks and for a low level monster it also has 120ft of True Sight which is pretty ridiculous.

EdIt: small addendum: If the PC's are 1st level, keep in mind that, depending on class and cons score, their HP will range between 6 & 17 and one critical hit might kill them outright so you might want to tweak the damage the monstes can do a little.

 There are plenty of low-CR aberration type creatures already in the game- Derro, Gazers, Dolgrims, Goon Balloons...  No need to try adapting old 3rd Edition templates just to get some level-appropriate foes.

https://www.dndbeyond.com/monsters?filter-type=1&filter-search=&filter-cr-min=&filter-cr-max=&filter-armor-class-min=&filter-armor-class-max=&filter-average-hp-min=&filter-average-hp-max=&filter-is-legendary=&filter-is-mythic=&filter-has-lair=&filter-partnered-content=f&sort=cr

I know, I have dug so deep into my books that it is kind of hard to find new things, I thought about a Derro Savant as a villain but scraped it, also I mean 1st to 3rd or something like that.