A creature it placed inside a Bag of Holding will suffocate. My question is, if said creature were left in the bag of holding after it expired, would it decompose, mummify or simply be dead with little effect, as if it were vacuum sealed and/or kept in cold storage? The temperature within the bag is obviously a relevant factor here as well.
My instinct is that it would mummify, but that would mean water vapor would leach from the body. Would it then begin to decompose? And what would happen to that water vapor?
Conversely, answer three here would indicate that rather than decomposing under normal conditions, the body would actually ferment. Again, temperature is relevant here.
To restate the question in more concrete terms: If a creature suffocated in a Bag of Holding and was left inside it, would an adventurer, opening the bag at some later point encounter a body that had 1) mummified, 2) decomposed under normal conditions, 3) fermented, or 4) not decomposed at all or done so very, very slowly?
Best Answer
It would rot
The rules for the bag of holding define the exterior and interior dimensions of the bag, what happens if the bag is damaged or turned inside out, what happens to living creatures inside the bag that need to breathe, and how the bag interacts with other objects that create/have extradimensional spaces.
It does not define any rules for preserving objects. Thus, because it does not modify 'normal reality', nothing stops the body from decaying as normal.
This is supported by sage advice, where Chris Perkins clarifies that the bag does not preserve the contents: https://www.sageadvice.eu/what-would-happen-to-a-severed-head-in-a-bag-of-holding-after-a-month/
Reality Check - But what about oxygen
If I place a dead body inside the bag, there is enough oxygen to give an average human about 10 minutes of air...plenty of oxygen to work with. Dead bodies do not consume oxygen.
In addition, oxygen is not required for decay. Anaerobic decay (decay without oxygen) is absolutely a thing. Decaying in this way would slightly change what molecules the body is broken down into, and occur over a longer period of time, but the body would still decompose. This would only happen if the bag was opened and closed in an environment without oxygen, or if the oxygen was consumed by a creature trapped inside the bag. It would also eventually happen as the body decomposes, and the oxygen is replaced with methane.