Exactly what it says on the tin. Kleeber the warrior is wearing a gambeson and carrying the standard adventurer's gear (leather backpack, 100ft of hemp rope, bedroll, spare clothes, waterskin, dry rations, and the 10 foot pole, some metal knicknacks like a spork, a knife, etc) and some weapons (longsword, dagger, shortbow, arrows).
Kleeber triggers a trapdoor in the Dungeon and falls into a 20 foot cube room that seals itself. The atmosphere in the room is pure oxygen and there's no light coming in. He uses a magical torch that doesn't need ignition, just a command word. That was a single-use enchantment and nothing else on him is magical. There is a pump sending more and more pure oxygen into the room. The walls and the pump are indestructible. The pump can't cause too much of a pressure shift in five minutes.
Physics and chemistry are the same as Earth. The torch is just a oil-soaked rag around a stick. Please note that the adventurer setup is just for ambiance. You could just as well put a redshirt cadet in a forcefield chamber with the same objects and the question remains the same so long the atmosphere remains pure O2.
Assuming that nothing else caused sparks before the torch is lit up, what happens in the first five minutes?