Hi!
Thank you for your question, normally phones do have a fail-safe in case of overheating which will turn them off after a certain temperature to make sure nothing bad happens with them.
A 15 hr gaming session is, while impressive, also pretty straining for the device in which it could be possible that something kicks in.
If the phone restarts even if it's not warm then I recommend checking with your closest ASUS Service Center so they can have a look at it
🙂https://www.asus.com/support/service-center-locationJust remember to eat and stay hydrated!