I am working out the characters for my trade caravan for my campaign and I have come up with one which is a failed apprentice who has run away to try and become a fortune teller. He has stolen a deck of cards from his master, which he is keeping safe because he "doesn't want the magic to wear out". He's entirely clueless and thinks it's going to let him read peoples fortunes, and it's actually a deck of many things.

The party will probably be around level 4-5 by the time they get to this. is that too early to introduce a Deck of Many Things to the game? Should I instead use a Deck of Illusions, which is less dangerous but still quite fun?

That seems fairly conclusive then XD

Either the deck of illusions or my own homebrew deck it is then (depending if I can get the homebrew deck done in time!)

you have 2 options

never 

go like gardmore abbey and trickle the deck through an adventure

I recently played in a game where the DM introduced the deck while the party was level 4. The player who found it decided that he was just going to bully NPCs into drawing cards. The next few sessions were hilarious.

I sometimes throw it in the treasure of the final villain. It easily can be campaign breaking, but I do think it’s fun.

I'm going to mostly agree with the others here, except to say that you could give it out if you want your campaign to be *about* the DoMT. Then it won't wreck the campaign because it *is* the campaign. Someone once described a campaign where the whole goal was to assemble the deck. Not sure who was making the party assemble it or why (well, I guess they why is obvious), or how it ended, but I think you could do something like that. Or give it out at the end of the campaign -- the heroes have slain Tiamat, and on top of the pile of her treasure hoard is a box, and in the box is the deck... The campaign is over, so whatever happens next is just for fun.

But don't introduce it in the middle of a campaign and think you can continue it as you have been going. That will never work.

Seconding what Lyxen (and stuffattackknightofrandom) said: back in AD&D era, several campaigns I was in were ruined (read: made incredibly awesome, but then ended out of necessity) by the full-fledged DoMT.  Certainly a lot of fun in the moment, but it definitely ended the campaign.

Later on (4e and 5e - link for conversion) I ran  Madness at Gardmore Abbey and had success both times (though the 4e group faded into the background as we transitioned to 5e).

Yeah, I'm not sure that the campaign is "ruined" in the conventional sense. It's just that once you get the DoMT in a party, then the campaign comes to be about the DoMT, and all other storylines will be dropped/irrelevant.

Whenever you want. Sometimes, dropping a powerful item in the collective laps of the party can lead to real fun.

I'm pretty much with Biowizard.  The Deck isn't a "game ender" per se.  However, if you have plans and arcs you want to do in your campaign (and it sounds like you do from other posts) don't do the Deck.  If you're got a truly open world (in other words less narrative structure than a sandbox, with a lot of places and people and things for PCs to go on demand) and your DM style is beyond "West Marches" in that the game follows the players actions there at the table, the deck is safer.  The deck is a catalyst, literally a game changer in terms of the characters' circumstances.  So if game night includes playing with the Deck, everyone has to cancel their plans.

If you want magicky card games that may tie more into your game, are you familiar with Tarokka?  It's a Tarot inspired fortune telling system that predates 5e I think but shows up in Strahd and I believe is talked about in the new Ravensloft book.  I'm pretty sure you can open source find the deck to print out but there are also prop versions for sale (I think WotC publishes the most price accessible one).  Tarot reading is sort of like good storytelling DMing, and Tarokka follows suit.  If you lean more to the improvisational storytelling aspect of DMing, and your group likes it, the Tarroka could be a fun way to present/foreshadow the possibly paths facing the adventurers (so it's not a game changer but puts some weight of fate into the narrative).  If you're looking for more Deck of Many Things mechanical consequences, Tarokka may not be your thing.  (EDIT: though I just saw a DMsGuild product that sort of treats the cards dealt a player as one shot boons that can be issued instead of inspiration.  I don't know if I like it, but the product is presently 70 cents, so...).

Real life Tarot decks are also fun to play with, and you have a much much wider array of art stylings to choose from (I've got a few, including a really nice one designed by H.R. Gieger which I want to use in something like the Wandering Emporium on Avernus or elsewhere in the Lower Planes if any of my parties ever gets there instead of say obsessing over their pet duck's welfare or perpetuating real estate deed fraud to acquire a base of operations for their detective agency (open world problems, ahem, players don't spy here do they?).  That said some folks get a little uncomfortable bringing props from RL occult/spiritual practices into their game so unless you know your players' comfort level in that regard probably best to stick with 5e Tarokka system.

If you think playing around with Tarokka might be cumbersome I'm sure you could find some sort of dice based fortune telling system that speaks to the tradition of "rolling the bones" as a form of fortune telling too.  

I know this probably takes the NPC from being incompetent fortune teller to competent fortune teller but again, unless you really want to throw a bomb into your game (after you've developed all these hooks and plans for NPC relationships) I'd avoid that and instead use the moment to draw the characters more into the possibilities of the campaign.

EDIT: If you want to just move away from fortune teller to a sort of magical dabbler who's basically a mess up, Bag of Beans is probably a bit more level appropriate chaos magic than the Deck, but then again that bean stalk possibility as well as a Mummy Lord possibility.  

Vince has some good examples of why you don't want to include the DoMT as part of a regular campaign, in the middle of it. You can't have those cards as possible draws, and expect the game to "just continue" the way it was after the cards are drawn.

Again, if the DoMT *is* the campaign, or if you've maybe come to the end of Curse of Strahd or Storm King's Thunder and the campaign story is over, and you want to have some crazy no-holds-barred fun before fully retiring the characters to start a new campaign, the DoMT could be a blast. But in any other context, it will utterly change the campaign from whatever it was before, to something wildly different and highly chaotic from that point forward.

The biggest problem/perk of the DoMT is that the consequences are essentially "permanent". A deck variant that was designed for shorter durations on the scale of an adventuring day/week might be a lot of fun and a great opportunity for adventure hooks.

Balance could temporarily turn a PC into a villain, perfect for injecting a little drama into the narrative.

Donjon would be a prolonged banishment, which would force the other players to overcome obstacles at a disadvantage for a brief window of time.

Temporary Buffs/Debuffs, conjured encounters, magic item that give the party an edge before blinking out of existence, etc...

Players could then choose to Play With Fate prior to a major event, and then let the outcome influence their approach.

I'm just putting it out there, but while I appreciate the prohibition against "wrecking" a campaign (which I think the Deck used in the above relatively early campaign with defined arcs would accomplish) there is room for "game changer" catalysts in an RPG.  When the game becomes a procedural exercise where everyone will eventually reach their desired optimum build at level 20 ... DMs and Players can get bored, especially if they've done that achievement in another campaign.  The Deck and similar wild random practices from prior games (like "you build a stronghold and the nature of your troops is determined by random table") are a time honored D&D tradition that lets us know that within RAW "flipping the script" can be ok.  If anything, I sort of like the random fortunes aspect of the Deck; but I'm also schooled in various arts use of "random chance" to reinvigorate practices.  Again, the Deck is unstable and the warning labels being read are accurate, but I don't think it deserves complete denunciation.  There's a place for the Deck, just like any game option.

As a DM I like it because I get to adapt my game too.

You can always leave the most problematic cards out of the deck or replace their effects with something that doesn't break the game.

You can replace cards.  This has already been suggested.  Which ones?  Half the deck can be a problem.  If you want the thing to have any real bite then there has to be risks, but that's terribly difficult with random stuff.  Many classes won't care if they cannot wear armor anymore, but the strength based heavy armor Fighter sure will.  If the deck is all hearts and flowers, nobody cares and why bother using it, and if it just gives bonuses then they are distributed unevenly.

I'd say the good time to use a Deck Of Many Things is never.

Which ones to replace are up to the DM. Having played in more than one campaign where the deck was introduced I have to disagree on it never being ok. It can be a lot of fun as long as the DM is aware of what it can do and the players understand there is a risk involved. Mileage may vary from table to table.

Quote from Lyxen>>

Quote from Buzzardoftheages>

And I don't think that anyone said "never". 

 Someone did say "never."

Quote from Geann >>

I'd say the good time to use a Deck Of Many Things is never.

I think Lyxen makes a good point about present-day players having different expectations of "agency" compared to players in 1e/2e days. Many players today expect to have a lot more control over their characters (up to and including deciding from level 1 which DMG magic items belong "in their build" and then expecting the DM to one way or another, at the appropriate level range, provide said items, as part of their "agency" in "controlling" their character) than we did back during AD&D days.

We have seen discussions like this break out on this very forum. A player not long ago came here complaining that his DM "had no right" to disallow the cleric and paladin in his party to recover spell slots when she viewed them as having gone against the precepts of their gods. The reason was simply, "it says I can do this on my character sheet, this is MY character, and the DM has no right to monkey around with what it says on my sheet." In AD&D times, I don't think many people would have made that argument -- it was readily accepted that clerics and paladins would only get back their spells on the condition that they were continually and actively pleasing their deity, regardless of what it "says on their character sheet."

If your players feel this way, and expect iron-clad control over their own characters (it's "their piece" of the game world), then the DoMT will not go over well. The DoMT can randomly and permanently take away control over a character sheet, or even permanently take away the character sheet itself. These cards would be viewed, by many modern RPG players, as "taking away their agency" -- their players' right to control their characters, as it were. Now, some of us old-schoolers would say, "Well, the decision to draw the card, or not, was where your agency factored in. If you didn't want to have a chance of drawing the insta-die card, you could have left the deck alone." But as Lyxen says, that is the old-school way of thinking, and modern gamers often do not feel that way.

This is why many of us say that the DoMT can "break" campaigns. Not only will it completely side-track the party from whatever story may have been going on at the time, but it will also do things to the PCs that modern players will not  like it to do. The odds are extremely good that a RAW-based DoMT will lead to frustrated players and a decision within a session or two of starting to make draws from it, that you should just abandon the campaign and start over (or else retcon the whole thing).

Buzzard has the only solution -- homebrew a DoMT with only non-game-breaking cards in it. Either take the most atrocious ones out (which is more than half the deck) or just custom-make your own deck that has minor or temporary effects rather than massive, permanent ones.