Spoiler for Fionna & Cake (Like I've watched anything more than clips)
I just saw a clip of what happened after the Lich's wish and it's just...so morbid. Happens in a flash. It shows this with Ice King doing his normal stuff, recording the roleplays he does with Gunther but he starts crying when it doesn't go right, then there's just a flash of his skeleton completely bare of skin and flesh, Gunter the same way in the corner. This is what happened after The Lich wished for the extinction of all life.
It's so dark, a sense of horrible unease. All life going about their normal day, and gone in a flash because of a creature who's only purpose is to destroy.
Just the thought of going about your day normally and then dying in a second without a chance to even perceive what's happening, and with everyone dying too there's no one left to remember you, is just so dam eerie.
And then after this, the Lich just gets depressed. This big bad that we've seen be a horror through the series literally develops depression, everything is worthless now that he fulfilled his goal and there's nothing and no one left to make new goals for.
But the worst part is just the feeling of complete and utter sudden hopelessness. Other than in the afterlife, just about everyone is GONE. Everyone in the Candy Kingdom including PB, Marceline, Lady Rainacorn, LSP, Flame Princess and her kingdom, Tree Trunks, Peppermint Butler. All dead in a flash, just bones or whatever was left, at the same split second Ice King and Gunther were nothing but clothes and bones.
And there's the possibility this is the timeline from the first seasons, which makes it so much worse. Meaning Finn & Jake ended up in a whole other home than their original, maybe.
Wish I could get MAX. I'm surviving off of crumbs on YT. But gods dam, Adventure Time really knows how to give you the creeps and existential anxiety.