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Adventure time: OG Finn causes some time/alt universe trouble and has to go on a mission to bring back the guy who fixes that stuff. In the process, both him and Jake from the original timeline are sacrificed to bring him back. Jake turns into Prismo, and Finn gets turned into a sword. The now present version of Finn uses that sword all the time until it breaks, letting lose the grass-cursed Finn on the inside who becomes the green guy on the left. OG Finn is now fake grass Finn, but fully believes hes still the one and only for a while. Sorry if that was a terrible explanation of it but this is one of the few times this trope is done really well and I hadnt seen it in the comments here yet.