Minecraft Fallen Fire

My brother used a command to have fire "powers" which activates when he holds a fire charge. When he uses it, invisible bats fly around (on fire) and make fire fall from a block beneath them.

The fire created a glitch and didn't get destroyed (some of it) and created a lot of lag for me (the owner of the realm). I tried to destroy it by hand, but it didn't help as he had also done it in the nether. Is there a command with which I could destroy it? If not how can I reset the nether?

Solution 1:

If it's just regular fire blocks, a /fill command should delete it:

/fill ~-20 -5 ~-20 ~20 ~5 ~20 air 0 replace fire

But you mentioned a more complicated command system. So if just removing the fire blocks doesn't help, you'll have to ask the command creator.