Ok, so man this is like really common but you don't see it a lot and that is really a shame!
INFORMATION
Classification: Suicide (Non intentional/Intentional)
Slow burn or quick: Slow burn or quick (Depends on external factors)
Time taken: No distinct time I can find, but likely no more the one to one and a half hours. It can also be instant.
Symptoms or Effects
• Bad Headache
• Dizziness/Light-headedness
• Red and hot skin
• Nausea
• Confusion
• Rapid Breathing
• Weak Heartbeat
Treatment
• Moving them to a cool place
• Drink lots of liquids (Mainly water or sports drinks)
• Fan them
Scenario
Characters: A, B, C (As the villain), D (Optional)
C has captured A, and forced B and D to come and find C, who is in (insert hot place here). C is defeated, and A gotten back, but as A and D celebrate, B lays down on the (insert floor of hot place) and dies.