So, there are plenty of allusions/paralles to Undertale in Deltarune and some easter eggs. I’ve made a list of the ones that I managed to find/catch. I’m pretty sure there are more of them.
1. There is an easter egg with naming your vessel and yourself. Like in Undertale using names of characters from the game will trigger a special dialogue (and like in Undertale naming your vessel or yourself “Gaster” will close the game).
2. When you look through the window in your room there is reference to Sans’ genocide fight.
3. A reference to Tsundere Cactus from Undertale, but in Deltarune it’s just an average cactus.
4. Recognizing yourself in the mirror.
5. Toriel hugs Kris almost the same way she hugs Frisk.
6. Alphys is still a weeaboo obsessed with Mew Mew ^^
7. Temmie still has the EG, but this time she is a partner not a proud parent.
8. If you try using your phone in the Dark World it will start making “Gaster noises”.
9. Ralsei’s cloak resembels the one that River Person wears (could River Person be a Darkner?)
10. Lancer’s bike might be a reference to Sans’ bike from the True Pacifist credit scene.
11. How Ralsei introduces us to the game mechanics is like a combination of both Flowey’s and Toriel’s tutorials. Telling what a soul is and how the fighting works is “Flowey’s part” and usage of Dummy and sparing being a preferable option is “Toriel’s part”.
12. Possibly a parallel to comforting a goatbro (Ralsei is a Goat/Boss Monster and a prince just like Asriel).
13. The shopkeepr hints a secret battle with Jevil in the early stage of the game.
15. The plush that Seam has on a shelf has is a reference to one of the Temmie Chang’s (she helped Toby create Undertale) characters, but I couldn’t find a picture of it.
17. The crown that K. Round wears might be reference to Super Crown, but the way it works is a ref to the big shroom power up. The sound that K. Round makes while growing up and shrinking is very similar to the sound from Mario games.
18. It is surely a reference to Sans’ room door (the weird glow), but it may also have something to do with Gaster’s grey door.
19. The sequence that Lancer and Susie perform is possibly a reference to Team R’s sequence from Pokemon series.
20. Possibly an Undertale Yellow easter egg? (the main charater is named Clover in UT:Y)
21. Even if you got rid of that tree you can still interact with it and get the dialogie box.
22. Lancer misses his attacks on Susie the same way Toriel does that when Frisk has low HP.
23. The theme that plays on floor ?????? inside the Card Castle uses the leitmotif of Gaster’s Theme. (can’t find a link with the theme)
26. You can interact with these pictures, when you interact with all 3 of them they’ll make a splat noise. I’m not sure if it does anything else.
27.
Lancer has the same weird pics in his room, you can interact with them too, but they won’t make splat noise. But on the other hand there are CDs with recorded splat noises (the same that 3 previous pics make).
28. She talks about the cake that Susie ate ealier.
29. A missing pixel lmao
30. The cards and chess are references to the enemies from Dark World. There’s also a plushie of Shopkeer Seam under the board.
31. Kris looks similar to unused Frisk sprite.
32. The word “library” is still misspelled.
33. Temmie still wants to go to colleg. That’s the spirit!
34. Apparently Alphys prefers Mew Mew 2 now.
35. Heats Flamesman reference.
36. Just Toby at work.
37. He’s slacking off however.
38. We finally know after which monster Gaster Follower 3 was modeled.
39. Deltarune is definitely a different universe/timeline, cause Undyne doesn’t know Alphys and she still has both eyes. Also Bratty and Catty not only barely know each other, but they hate each other as well (in Undertale they are best friends).
40. On the left – dude who bought Muffet’s donut and regreted that later on, on the right – that weirdo from Napstablook’s snail farm.
41. Police station and Undyne’s house share the same doormat.
42. The big mouth monster that was spending time at Grillby’s, also a possible reference to nurse Joy from Pokemon series.
43. A reference to the absurdly tall sink from skelebros’ house. (also that blue bear might be inspired by 5.0.5 from Villainous)
44. Gerson is dead in Deltarune, also his last name is Boom. The hammer on his tombstone refers to his “nickname from war times” from Undertale – Hammer of Justice.
45. The tombstones of monster who were amalgamates in original Undertale, but since in Deltarune Alphys was never a Royal Scientist, all of this monsters just died. From left to right – Snowdrake’s Mother, the dog (Endogeny), Shyren sister (Lemon Bread).
46. Hand Receptionist from MTT Resort and politics bear from Snowdin.
47. That weird cube-dog-like monster from Sans’ crossword.
48. A reference to Purple Guy from game series FNAF (in Deltarune Burgerpants works at Pizzeria after all, hello?! it must be it xD).
49. A bunny shopkeepr from Snowdin, the guy who works in Core, the Ice Wolf (and he finally has pants as he wanted), Fuku Fire and Skateboard girl (but they already graduated), the lion monster who is a fan of Mettaton and gets a dress from him after his musical, and the dragon dude who couldn’t return to his family cause the elevator at MTT Resort was turned off.
50. Sans’ store is literally Grillby’s bar, but the name “Grillby” was removed and Sans added “ans” to the letter “S”. But he didn’t remove apostrophe. Well done Sans, well done.
51. A reference to Spider-Man, “I’m your friendly neighborhood Spider-Man”. Sans even calls Kris “tiger” which is how MJ calls Peter (I forgot to make a screenshot of it tho).
52. F#ck you Sans xD
53. “trousle of bones” – it obviously refers to Papyrus’ theme Bonetrousle.
54. Little bunny in the window is a child of that bunny monster who runs Snowdin Inn.
55. A monster from Snowdin who loves knocks lives there. And apparently Kris is not as good at knocking as Frisk is.
56. In the alley where you meet Alphys again there is a painting of Everyman (attack of Reaper Bird – one of the amalgamates). You can’t escape from your sins Alphys, even in another universe/timeline.
57. Asgore’s introduction dialogue in Deltarune is literally the same as in Undertale.
58. These flowers are an obvious allusion/parallel to the souls containers. The golden flower in the middle might be representing the empty container for Frisk’s determination soul. If you remember, in Undertale Alphys used a golden flower as a empty vessel to inject determination and that flower became Flowey later on.
59. RG01 and RG02 are still together, and for some reason they are still wearing helmets even tho the Royal Guard is most likely not a thing in Deltarune xD
60. If you wait long enough by the lake Onionsan will appear. He will ask what’s our name and will want to become friends. He also doesn’t remember his own name due to being alone for too long, so we have an option to gave him one.
61. He also might have a dark secret hehe
62. So there is most likely Mettaton, but we have no idea what’s up with Blooky. I’m hoping that in chapter 2 we will find out Mettaton’s canon ghost form and maybe his real name. Also I doubt that in Deltarune Mettaton has machine body, since Alphys never was a Royal Scientist in this universe/timeline.
63. Chara possession much? also a hint to the ending of Deltarune chapter 1
64. After Kris locks his soul in the cage you can still move it, but you can’t escape.
65. The glowing red eyes, Post Genocide Pacifist vibes are strong in this one.
These are all the allusions/parallel and easter eggs I was able to find. I’m sure there are more. Also there are some that I forgot to screenshot such as “characters titles” in party menu change: – Kris has a title of Leader, but if you check all 4 beds in Card Castle he has a title of Bed Inspector – Ralsei title changes after giving him different scarf There’s also at least one Annoying Dog easter egg in the party menu.
Not to mention the anagrams: Deltarune – Undertale Ralsei – Asriel
Procrastination Queen 