The party rogue found a suit of armor that happened to have an evil soul bound to it. He unwittingly stuffed it into his bag of holding to sell the suit of armor later. The animated suit of armor does not need to breathe and to my understanding is floating about inside the bag. I was planning on having the suit attempt to open the bag from the inside and climb out when the party was sleeping.
Is this legit?
According to the item properties in the DMG, no, it can only be opened from the outside. Specifically, items inside require an action to be retrieved.
(To satisfy those who have asked: the word retrieve, as used here, is a transitive verb with an implied indirect object, usually the same as the subject. The direct and indirect objects of a transitive verb can very rarely, if ever, be the same in English. Therefore, no, something can't retrieve itself, because that doesn't make sense.)
However, YOUR bag of holding does not have to be the bag of holding presented in the book. If you say that it can happen in your campaign, then it can. Never let minor mechanical details get in the way of the story (being mindful of your players, of course).
Nothing supports this.
Players' expectations of an item are in keeping with the knowledge available about said item. If a bag of holding is modified in such a way as to allow opening from the inside, it's something players should be aware of, specifically so they can guard against scenarios presented like the one you have illustrated above. To elaborate slightly, this isn't to say your players would know about every magical item in every way, but a bag of holding is fairly common and is something every adventurer gets their hands on as soon as possible.
For those asking about the source for this:
DMG Pg. 153-154 Bag of Holding
The description does not provide that the bag can be opened from the inside. The picture also provides visual cues that indicate the bag is latched shut in some fashion from the outside (this is likely due to prevent it from being accidentally filled with items that would exceed it's 500 lb weight limit and destroy it and your contents, for example if you were to fall into a river.)
As always in 5e, and since every answer seems to require this as a repeat: Rule Zero trumps everything.
A more descriptive (yet ridiculous) name for "Bag of Holding" would be "Travelling Portal to Airless Demiplane Storage", or TPADS for the hip kids out there.
I suggest this because my interpretation of the Bag of Holding is that its mouth acts as a portal into a 64 cubic-foot demiplane. When you open the mouth of your nifty Bag/TPADS, you open the portal, and you can use your action to retrieve a single stored thing via fancy magics which prevent you from bumbling around, grabbing stuff indiscriminately. When the mouth is sealed, the portal becomes inactive, or at least blockaded.
Why a demiplane instead of Astral Plane? Because whenever, as a result of damage or conflicting magic, a TPADS' contents are disrupted and need a place to go, and they go into the Astral Plane, then they probably weren't there to begin with. Otherwise you'd have a bunch of storage-locker bubbles floating around the Astral Plane, being knocked around by Gith slave-ships like billiard balls ("Dude, Where's My Astral Bubble?"). Hey, it's better than annihilation.
The devs anticipated placing creatures into your handy TPADS, and so created the parameters for suffocation [DMG p.154] while within its confines, perhaps unintentionally encouraging munchkins everywhere of its use as the lazy adventurer's alternative to waterboarding. Presumably, a creature(s) which requires no sustenance whatsoever could survive the demiplane indefinitely, as is inferred by the rules for breath-sharing (Bag o' Zombies - Beware!). The inclusion of suffocation rules infer that creatures trapped inside cannot escape due to an interplanar barrier blocking airflow and most likely light and passage as well.
You can't TPK a party with ancient accursed-armor, grimed by generations of malice-crust, if it's eternally imprisoned at the mercy of the adventurers!
How would a fully-immersed, unwillingly trapped creature escape from within such an anaerobic demiprison?
What if an overly-ambitious party of adventurers attempt to Trojan Horse a Lich with a TPADS marked "499 lbs. of milk & cookies!", ignorant of the Lich's disdain for dairy and confectionery (classic n00b error), and its subsequent dismissal of the gift?
The description of this item only mentions what happens when exterior forces are enacted upon it. I shall provide two explanations in favor of escape, because OP's encounter idea of PC's being ambushed by their own backpacks needs to happen early and often (queue evil DM laughter). These scenarios depend upon how you envision the interior portal, although you'll have to use your mind's-eye because there's no light inside a TPADS.
In this scenario, a creature could manipulate the interplaner bunghole from the inside out, much like a toddler rogue-in-training would do after becoming trapped inside her parents' footlocker. Whether or not this requires high-level conjuration-based travelling spells, or more mundane methods, similar to firemen spreading open a busted elevator compartment, is your option. Just make sure to solicit at least one pizza-bribe before making a final ruling.
Under this interpretation, the trapped creature would have to lie in ambush, awaiting the portal's activation, triggered by the external mouth being unsealed. Whether the in / out portal activates from the exterior mouth being fully opened or merely incompletely closed is yet another delicious pizza-bribe opportunity (and I don't want to bias readers by only including pizza in select answers)
Now, I know what you're thinking about #2, or at least what your players will whine about.
Didn't said Wizgineers (TM) design the TPADS so that my character never touches the wrong content(s) - something can't touch me unless I want to touch it, can it!?
And to this, I say: probably! Most Wizgineers (TM) are as concerned about product liability as the next magitechnophile. Thus the most accurate way to do #2 (huh huh) is to wait for the adventurers to voluntarily reach for their own doom before TPK'n them - MUAHAHAHA!
Or, you can just rule that everything inside the bag is in a pile, and just because the thing you want is on the top every time doesn't mean that something mobile can't move into contact with the reaching arm. Perhaps the cursed armor floats near the portal, preparing to dash outward or lay a latent curse upon any arm foolish enough to enter its sacred domain. To arms!
{note2self: place Bag of Holding in dungeon filled with traps, hazards and/or curses and cackle mischievously when party turns it inside out}
Malice-encrusted bunghole is the lazy adventurer's alternative to milk & cookies
Yes
The description specifically mentions the possibility of the bag being overloaded or pierced. As such, a living suit of armour with lots of pointy bits should have no trouble escaping (into the Astral Plane, as pointed out by the dark wanderer in comments).
It honestly depends on your group in a lot of ways. I've played in groups where rigid adherence to the published rules is very important to gameplay. They don't care for surprises, and so deviating from "the standard" can be distracting and unpleasant. One of the major jobs of the DM is to make sure the players are enjoying themselves, so if your group is a "rules" sort of group, then I'd say the answer to your question is a swift "Definitely not."
On the other hand, I have also played with groups that tend to bend the rules and enjoy a good surprise every now and again. They would love to see that magical suit of armor break out of the BOH and do something crazy! They're just those sorts of people ;).
In the end the answer to the question behind your question is this: Talk to your group of players and figure out how they feel about you bending rules without warning them about it. Do they think it's a nifty idea? Or do they think it would detract from the experience? Once you've got that answer in hand, I think this question sorts itself out.
Just stumbled on this response from Jeremy Crawford on Twitter where he gives his personal, unofficial ruling:
A bag of holding can be opened from the inside, unless it's been sealed shut somehow. https://twitter.com/JeremyECrawford/status/1034580941383954433?s=20
That doesn't totally complete the solution, but it does imply that a Bag of Holding isn't assumed to be sealed shut, and that effort must be made to keep a creature within.
The bag does not explicitly talk about this in any way, so the DM must decide.
The bag merely says:
Retrieving an item from the bag requires an action.
Retrieving an item is something that only applies to someone outside the bag. A creature inside the bag is not being retrieved if it tries to open the lid and climb out. In logical terms, this only does restrict how someone outside of the bag can get an item out. It's like saying "unlocking the door requires a key" -- sure but someone else might just batter the door open, key or no key.
Breathing creatures inside the bag can survive up to a number of minutes equal to 10 divided by the number of creatures (minimum 1 minute), after which time they begin to suffocate.
The other supporting factoid for the view that someone inside cannot get out is that a breathing creature suffocates in the bag if it stays too long. And if it just could open the lid and climb out, it would not suffocate, which pretty much any creature will try to avoid, so what good would that sentence be if it could just get out? But again, from a perspective of logic and of what this says, this does not prevent a creature from climbing out. It just says that it dies if it remains too long, not that it must remain. There may be other reasons why the creature does not want to move out, for example to smuggle it by some guards, and if it comes out too early, it will be captured. So the limit has meaning, even if the creature could climb out.
Wether you can get out by damaging/destroying the bag from the inside is another question -- and also up to the DM.
