The Deuterium 200 is a liner bag designed to connect to the inside of the Torpedo and Hellfire sleeping bags in order to boost the temperature performance by a season (approx 5-7°C).
Crux says:
Not only is the Deuterium 200 a low-cost alternative to buying a new, ultra warm sleeping bag for what may be a once-in-a-lifetime expedition, but it can also be used on it's own as a lightweight, warm weather bag for summer trips.
Although Crux's Deuterium 200 Down Liner Bag is designed to fit perfectly in the Torpedo and Hellfire sleeping bags (attached using the side toggles) it can also be used in any other mummy sleeping bag, just without attaching the toggles.
A really good idea, having a versatile liner and summer sleeping bag in one.