How do I make fire form if a specific splash potions lands?

It is actually quite easy to do! Just put this command in a repeating command block and power the command block when you want to activate it. /execute at @e[type=minecraft:potion,limit=1,sort=nearest] unless block ~ ~-0.7 ~ minecraft:air run fill ~-1 ~-1 ~-1 ~1 ~1 ~1 minecraft:fire replace air This command will make all potions to Molotovs! That's why you should turn it off when you don't want it :)

It is more advanced if you only want a specific potion to be like a Molotov! (I don't know how to do that right now but you must do something with NBT tags!)