The short answer
You burn it — you don't fight it. Players in the Steam thread "how to burn the grain rot on the 2 floor" describe three different ways to get fire, not one.
Method 1: Possess the firefly bot
"find the firefly bot (the one that acts as a flamethrower) and posses it. You now have the means to remove the grain rot."
This is the method most players name first, and one described it simply as "FireBot was easy to use."
You are a spark inhabiting vessels, and different vessels have different abilities. The firefly bot is the one that makes fire.
Method 2: Craft lanterns and smash them
"just craft lanterns and smash them or grab the desk lantern"
No possession required. A related warning from the same thread suggests the mechanic is about releasing the flame:
"if you got the lantern construction it mentions turning it off before disassembling. I'm thinking that if you break it before turning it off it might spill flames everywhere"
⚠️ That last part is a player's hypothesis, flagged as untested by the person who wrote it. The lantern method itself is reported as working; the "why" is inference.
Method 3: Upgrade your shop for a flamethrower
"either the fire bot, put it in the elevator to bring it up or level up your shop for flamethrower"
A permanent solution rather than a per-run one.
⚠️ We do not know which shop level unlocks it. Another player asked exactly this in the thread — "what level for the flamethrower? my shop is level 5 or 6 currently" — and nobody answered. If you know, that is genuinely missing information.
Bonus: you can take the fire bot home
The same thread contains a core extraction rule that is easy to miss:
"anything loaded on the elevator comes home with you, even loose gear, furniture, constructs. It doesn't have to be possessed to come home, just on the elevator when you ascend."
So the fire bot can be carried up rather than used and abandoned.
What is not confirmed
- Where the firefly bot spawns — the dungeons are procedurally generated and shifting, so a fixed location seems unlikely, but no player report states this either way
- Which shop level grants the flamethrower
- Whether the rot regrows
- Whether bots brought home persist across future runs (a player raised this exact question in the thread and it went unanswered)
Watch your spark timer
Possessing something means leaving your own vessel, and your spark runs on energy. If it runs out while you are disembodied, you cannot get back into your original body — and your original body breaking is an instant game over.
Players describe using lights in the level to temporarily stop the spark timer, which matters if you are hunting for the firefly bot rather than walking straight to it.
Park your own vessel somewhere safe before you go looking.