Keromnasol

A poison that is an odorless clear liquid.
Makes you tired and lethargic.
Makes you fall asleep, eyes closed, but still conscious. Restless, like you are trying to fall asleep, but you can't.
Slowly, your muscles stop working, and your body temperature rises.
Once you can no longer move, and it feels like your body is boiling, from chest to limbs (the temperature isn't painful, a little too warm, like your clothes just got out of the dryer, and you put them on, but your skin is that hot), or at the point where you "give up", you die.
You choose when to die, but ultimately, you will die. It often takes an hour to finally accept death.