The North Face Dark Star sleeping bag is guaranteed to keep you warm. Why? Because it weighs SIX POUNDS (add another half pound for a size long) and you will benefit all night from the heat you generate just carrying it around.
Rated to -40F this bag is seriously toasty and will help you win the battle against the z-monster in the most frigid of conditions. The weight is due to the synthetic insulation, Polarguard Delta in this case, rather than lighter down. My climbing partner, who happily trades weight for warmth, could not make himself cold in this bag at temps near the -40F rating.
The usual TNF quality with a soft brushed lining, watch pocket, draft collar, all that. Packs up to the size of a medicine ball, which function it will perform admirably. I’m not sure why you need synthetic at -40F, since all precipitation will be frozen anyway, but if you do and you don’t mind the space and weight this is your bag. They are cut a little roomy.