There is zero documentation on "Dark Wake Thermal Emergency" apart from a reference to it being a reason for making the computer sleep.
We understand it to mean that the computer was suffering from a very busy insomnia, it reached a point where the computer got too hot and went back to sleep to prevent causing damage to itself.
We believe this to be a side effect of long insomnia, in our own labs we've seen this shortly after an Adaptive Standby Wake cause the machine to be awake for half an hour or more, while ambient room temperature was above 30˙c.
As this appears to accompany a Adaptive Standby Wake, we'd recommend following the suggestions on that page to resolve "Dark Wake Thermal Emergencies".