>>93911855that's actually a really complicated question but I'm bored so I'll go for it.
Finn got a Cursed Grass Sword that was attached to his arm
the arm it was attached to gets ripped off
finn goes armless for a bit
turns out the curse never really left him and the sword turns into a new arm
Finn gets a sword that is himself through a paradox (put in simplest terms)
Finn sword gets cracked and during a fight, the grass curse leaves Finn for the sword turning into the grass creature he is fighting in the gif (who is now a mix of the grass curse and Finn's memories) [called Fern}
Finn gets a robot arm from Princess Bubblegum
Fern doesn't know how he fits into the world and just sees himself as a copy of Finn
Fern tries to be his own person but gives up and instead decides to try to take over Finn's life by locking him in a dungeon
Finn escapes and catches up to Fern and queue gif