I like to give my players bags of holding pretty early on, because I don't like worrying about if the have room for stuff of marking where on there body it is. If it's not specified it's in the BoH. However what I was not expecting was a character who rolled a nat20 history check on the deck and therefore was granted that his character knew what all the card were (but not where in the deck they were) she but the deck in her BoH and because a bag of holding has what ever you desire at the top, said she was going to draw the three of cups out of her bag of holding... I rules this as acceptable because it was super creative and the other players took the BoH for granted and ignored it. How would you have ruled, and what other ways have you seen the DoMT get hacked?

I think that is some pretty clever thinking on the players' part, but I would not have ruled in a way that would have favored the scenario going down smoothly.

Firstly, I wouldn't have had an ability check provide such certain information whether it was a natural 20 or not - legendary items, in my view, should only have legends told about them - not reports - and legends, unlike reports, are often wildly incorrect in some way or another.

Secondly, the deck being legendary and the bag not, I would always lean in favor of the more potent magic when picking which item's particulars take precedent - so the deck's requirement of choosing how many cards you will draw, and which cards those are being determined randomly, would completely supersede the bag's ability to draw out what you want (which isn't actually what the item says, according to how I read it - it's Heward's handy haversack that says the item you are looking for is "magically on top" - but still, the item in question being removed from the bag in either case would be the whole deck, not one specific card of one's choosing).

But that's just me not wanting to get player's thinking that a clever idea assures something works regardless of sense, or that the difference between doing normal things and doing things which are declared clever can be so great as to effectively render a legendary and dangerous item like the deck of many things a near-infinite source of no-risk bonuses.

Yeah, the item in question is "the top card of a deck of many things, which I hope to be the three of cups". Being such a magically powerful item, simply picking out the three of cups would have no effect; it has to be the actual card drawn from the actual deck, and the card drawn from the deck is always random. If anything, you could say they pulled the card, but without having drawn it randomly from the deck there's no effect, and now that deck no longer has that very useful card.

This is exactly how I'd rule it. Yes, you absolutely pull the card you want from the Bag of Holding. No, it does not count as a draw from the Deck of Many Things. The magic of that item requires a blind and completely random draw. Circumventing that randomness circumvents the magic (where circumventing = short stacking the deck).

The Deck of Many Things is like nuclear war... The only way to win is not too play.

:)

WoW that's very creative! You're kinda stuck because that's the sort of situation where you have to decide something on the spot and stick to it.

If this comes up in my game (the DoMT is sitting right in my notebook waiting for them to find it!), I'd like to say something like "you can try that"... "you don't know if that will work the way you intend or not"...

After that, if they still go with it, I'm not sure what I'll decide, but I'm going to think about it now so that I have a response! I'm inclined to say that it doesn't work that way. But then again, it kinda makes sense... So, I'm probably inclined to roll some dice (say a 50/50 chance?) to see if it works in their favor. And, if not, then oops... every single card from the top down to the one you wanted are all flipped over at the same time! Yay DM!

If I recall I think natural 20s are only automatic in combat. Though I could be wrong. 

The way a bag of holding works is what you think of is what comes out. So if you knew all the cards you could potentially draw them out one at a time. Unless of course the bag required you to remove the deck because it is one thing. 

However I would probably rule that removing each card would prevent the decks magic from working. 

Reward the player xp for clever thinking. But not allow the deck to be abused in such a way. 

I would agree however, the only acceptable way i would allow this , is if they were creative enough to only pull the cards they wanted from the bag of holding, and then reconstructing the deck with only the cards they desired, a real "ship of theses" scenario. if you assemble all planks of a ship free from the rot is it still the deck of many things.

Short answer yes, they simply reconstructed the deck with the cards they favored and as long as they shuffle it. the cards are random but not known to them.

If they managed to do that I would give them inspiration.

I would have the ENTIRE deck come out.

If you let them remove the 3 of cups that way without activating, their next move would be to pull all the negative cards from the deck, put them in a portable hole or other magical device.

Then pull the deck out of the bag of holding that now has no negative cards and draw randomly from it.

Only pulling “the good/bad cards” from the bag of holding would require meta knowledge about the deck and which cards are harmful. The Players may have access to that information, but their PCs shire as heck wouldn’t. So how would they know which cards to call for and which ones not to call for? They wouldn’t. They would have to call for the whole deck.

Lots of ways to get that information, assuming a Deck of Many thing is not an Entirely newly invented magic item. 

Even if they only get a few of the bad cards out, it goes from a bad idea to a good idea.  If you get rid of these four ( Skull, Euryale, Flames, and Void), then drawing from the 13 becomes a 2 bad but not horrible out of 9 card deck.

The Deck of Many Things is a legendary magic item. As a DM I would require a quest to ascertain that information.

1) Kudos and bonus XP or an Inspiration Point to the player for the clever idea. You don't want to discourage clever play. If they get some sort of reward, it might encourage the rest of the party to try and think outside the box.

2) The Deck AS A WHOLE is a Legendary item...not the individual cards. Therefore, I would rule that you cannot draw a card from the deck while the rest of the deck remains in the BoH. You would first have to remove the deck and THEN draw the card, thus neutralizing the BoH's influence over it.

3) Because of the clever idea (going back to #1), I would explain all of this to the player and then ask if they wanted to remove the deck and then draw the card(s). If they did, I would grant them ONE Mulligan for their clever idea (a very tangible reward if the first card goes badly). If they draw a card and it's bad, I would simply say 'This is not one of those cards you were hoping for. Would you like to take your Mulligan now?' If they said yes, the card goes to the bottom of the deck and they may draw again.

In this way, you're rewarding clever play in a very meaningful way and yet not removing ALL of the risks of the deck. You're also not setting a dangerous precedent for the BoH where the party can put an item INSIDE a container, put that container inside the BoH, and then use the BoH's special abilities to draw exactly what they want from inside something.

Something to remember though; If you do this and the player draws a good card from the deck and stops, they STILL have their Mulligan. They can use this for a similar situation (random thing where the result can be good or bad) to avoid the FIRST result if they don't like it.

Good luck!