Fun fact: my first contact with undertale AUs were from underverse. That meant the first AU sans I saw that I knew was from an AU was Cross (the first AU I've seen is outertale but I didn't realize it was an AU)

So here's a complete reaction of a completely confused me trying to understand to concept of AUs while the plot of the story made it even more confusing. In addition, I watched underverse before watching undertale genocide and thus had no concept of sans and resets so yeah,

It was a hell of a binge watching

Starting with sans appearing in a white background
Me: wat

Cross/Chara: *appears*

Me: um..... Hi AU guy???

Ink: *appears*

Me: who the flip are you wait you're cool I like it

Ink: *vomits ink*

Me: *accepts this as canon as I saw nothing else of ink before*

Ink: *talks about error and core frisk*

Me: (at that point doesn't know who is error and paid no attention to frisk) woah that sans is mean

Nightmare: *appears*

Me: woah inky tentacle skeleton

Dream: *hi haha bye*

Me: oh hi

Yeeted to underfell
Me: woah why does everyone look so mean (at that point I didn't know the word edgy existed)

UF!paps: I'LL SERVE SPAGHETTI WITH YOUR SKULL

Me: PAPS IS SO MEAN

Core!Frisk: *appears*

Me: *confused human noises*

Yeeted to underswap
Me: why is everyone in the comments so hyped about this AU?

US!paps: *appears*

Me: wtf happened to papyrus

Later...
US!sans and US!Chara: *appears*

Me: how cute!

*Cue the convo the two had on the hill*

Me: OH GOD THIS SANS IS DOWNRIGHT ADORABLE AAAAAAAA (I'm not exaggerating I genuinely found canon swap sans adorable)

This part starts getting confusing cause XGaster and the overwrite concept comes out but I couldn't understand a thing so I had no interesting reaction

*Cue the huge fight in underswap*

Me: OH GOD STOP TORTURING THE POOR BABIES

*That very tear jerking part when blue cries his eyes out while being dragged to the portal*

Me: *crying eyes out as well*

Yeeted to Outertale
Me: wait this AU is so pretty I love it

*That huge ink and error fight*

Me: woah woah woah WHAT IS HAPPENING INK WHY DO YOU LOOK SO COLD OMG WHO'S KILLER WHAT WHAT WHAT

Error: *destroys all AUs*

Me: WOAH WOAH WOAH CALM DOWN THERE GUYS

At some point XGaster appears but as always I didn't have a good reaction cause I was seriously confused
XGaster: *kills blue and red*

Me: WAIT WHAAAAAAAA

And that's it for my insane reactions!