I have also made achievements for Deltarune and Undertale Yellow!

they're bad.

You can check them right here
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3130937851

This guide contains spoilers for the game but story achievements are hidden.
Story achievements are hidden like this ya like jazz?. Miscellaneous, Challenge and Optional achievements are not hidden.

• The Tale Begins - Name the fallen child and start your journey
  
• Friend or Foe? - Meet Toriel
  
• The Ruins - Leave The Ruins
  
• Welcome to the Underground! - Meet Sans and Papyrus
  
• Snowdin - Leave Snowdin Town
  
• Swordfish - Meet Undyne
  
• Waterfall - Leave Waterfall
  
• awkward - Meet Alphys and Mettaton
  
• Hotland - Leave Hotland
  
• Showtime! - Meet Mettaton EX
  
• The Truth - Learn the truth about human souls
  
• The Core - Leave The Core
  
• JUDGEMENT - Get judged for your actions by Sans
  
• Regret - Meet Asgore
  
• Nightmare - Meet Omega/Photoshop Flowey
  
• The Underground - Escape the Underground[/list]

• A Bone Chilling Date - Go on a date with Papyrus
  
• Hearts in Flames - Go on a date with Undyne
  
• Trash Date - Go on a date with Alphys
  
• Revelations - Learn about the experiments
  
• True Lab- Leave the True Lab
  
• Angel of Hyperdeath - Meet Asriel Dreemur
  
• Hero - Save your friends and escape the Underground[/list]

• Ruined - Exterminate The Ruins
  
• Kill or be killed - Kill Toriel
  
• Avalanche - Exterminate Snowdin
  
• Trust no one - Kill Papyrus
  
• Inundation - Exterminate Waterfall
  
• Sushi - Kill Undyne The Undying
  
• Disappointment - Kill Mettaton Neo
  
• Meltdown - Exterminate The Core
  
• Bad Time - Kill Sans
  
• Lost hope - Exterminate all of Monsterkind
  
• That feeling... - Meet Chara[/list]

• In a Hurry! - Finish the game in under 2 hours and 30 minutes
  
• Fast and Clean Kill Sans without healing
  
• Weak - Kill Undyne the Undying without getting hit
  
• YOU LAZYBONES! - Beat Papyrus without moving left or right
  
• Thank you! - Finish the game with the snowman piece in your inventory
  
• Extrovert - Interact atleast once with every monster in the underground
  
• Introvert - Don't interact with any monster in the underground
  
• Hungry for more - Beat hard mode without taking damage
  
• Puzzle Pro - successfully complete mettaton's tile puzzle
  
• Dogveloper - Dodge all the credits at the end of the True Pacifist route and open the secret door.

• Merciful - Spare a monster
  
• Ruthless - Kill a monster
  
• Chicken - Escape from an encounter
  
• Determined - Come back after dying
  
• Undertale - Finish the game
  
• ? - Experience a FUN event
  
• Are you serious? - Beat hard mode
  
• oh. - Die To Your First encounter (Doesn't Include The Dummy And The Froggit Toriel Scares Off)
  
• Trust Issues - Get a betrayal kill on Toriel
  
• Pet Overdose - Pet Lesser dog until his head goes off screen
  
• Loot-Goblin - Get every item in the game at least once
  
• Dance - Do the dance
  
• Good Doggo! - Play fetch with every dog enemy
  
• Good Skelly! - Use the stick against Papyrus during his bossfight
  
• greM - Get hit by every type of enemy in the game
  
• Problem solving is my passion - Fail every Mettaton minigame
  
• TEM VILAGE!!! - Find de tem vilage!!!
  
• TEM GO TWO COLLEG!!! - Pay four Tem's colleg!!!
  
• INVINCIBLE!!! - Get an use Tem's armor!!!
  
• I still love ya! - Take 10+ hits in a battle, and then spare the monster(s)
  
• Extinction - Kill every single enemy during a genocide route (including secret ones)
  
• Patience Is A Virtue - Encounter and Spare/kill Glyde
  
• I'm So Sorry! - Encounter So Sorry
  
• Wait for it! - Spare 3 monsters at the same time
  
• We'll be here for a long time =) - Quit the game without saving right after killing Sans
  
• Souless - Don't hug Asriel
  
• Glutton - Eat 3 items while having max HP
  
• Making that dough - Have 10k Gold at once
  
• Bing spender - Spend 5k in a single shop
  
• Waste of my time - Kill Jerry
  
• Musician Fingers - Complete the piano puzzle in Waterfall
  
• Statue - Beat Doggo without getting hit (or hitting him, you monster.)
  
• Bone to pick with - Get access to Sans's room
  
• The least you can do - Exit the underground with all the armor or weapon items of the previous humans
  
• Pink is the new black, er, yellow - Replace the yellow names with pink names
  
• Tra la la - Ride on the river person's boat
  
• Tre le le - Hear half of every dialogue that the river person has to say
  
• Tri li li- Hear about the man who came from the other world or speaks in hands
  
• Weed Killer - Avoid Flowey's 'friendliness' pellets
  
• Not so bad after all - Pick up the crossword paper during a Genocide run
  
• Slacking off - Stay immobile as soon as you fall for more then 10 minutes
  
• Unoriginal - Type the name of a main character when asked about the fallen child's name
  
• Trusty Hardware - Finish the game without equipping any different armor or weapon
  
• Cooked Salmon - Abandon Undyne when you reach Hotland
  
• Woah.. - Lay down with Napstablook
  
• Too slow - Lose Thundersnail
  
• Turbo - Win Thundersnail

I personally don't count these as achievements that count towards completion because they are either too hard or time consuming so if you're a completionist and are using this guide for, well, completing the game, then you don't have to do these. Unless you're a masochist.

• Not the only one who can dodge - Kill Sans without getting hit
  
• Time to SPARE and KILL - Beat True Pacifist, Reset, and then do Genocide, IN ONE SITTING
  
• Don't you have anything BETTER to do? - Get every ending! (Includes Pacifist, Genocide, All 93 Neutral endings)
  
• Not The Only Pacifist - Beat Pacifist With No Damage
  
• Are You REALLY A God? - Beat TRUE Pacifist With No Damage
  
• Ok, Now You're Just Flexing - Beat Genocide With No Damage
  
• True Determination - Finish a Genocide run without saving
  
• Lots of FUN - Experience every FUN event atleast once
  
• Dog of Doom - Get the Dogcheck room for your first time
  
• Bored??? - Read every single line of dialogue possible in the game
  
• Missed the bus - Beat the game in World Record time (1.02+, 55m 15s)

Q - What happened to the Deltarune achievements?

A: I think they're really bad and rushed so I murdered them all.

If you have any achievement ideas you can send them in the comments and, if it's good enough, I will add them to the list. (You can even send ideas for deltarune achievements so I can maybe remake it to be good! No promises, though.)

And don't forget to like and favorite (or even give an award!) to the guide as this helps it stand out over other guides and give more engagement! (I sound like a youtuber lmao)