Thats a great name
This might be the most iconic magic item from all of D&D, and yeah, it makes for great stories
To be fair, D&D purists will point out that the card pulled in the image is obviously the Skull, which in D&D pits your character in a one on one fight with an avatar of death itself (or a simple skeleton if you're a boring DM, I guess). I should probably tweak the card to reflect that. Losing half your life, rounded up (or just a fixed 7 or 10 life so it can actually kill you) is a more appropriate fit.
Also, creating the Knight token is fun and all, but it's kinda lame that you only get to attack with it twice if you take it at the first opportunity. Maybe it should need to get haste until end of turn.
Other positive options from the deck of many things
Comet: In D&D levels your character if you defeat the next monster single-handedly. Hard to translate to MtG.
Jester: In D&D gives you a buttload of experience points and you can draw extra cards from the Deck. Easily translated to "Draw two cards", but that ability already is on Demonic Pact, so that's a bit boring. Besides, I like the shuffle something back and draw a card better on this this 3-drop I'm creating.
Key: A rare weapon (that you can actually use) appears in the player's hand. In MtG this should obviously fetch equipment from you deck. The problem is that there isn't that much equipment in my cube, so this would end up being a really narrow (or even useless) ability. I could have the card create an equipment token, but that is way too wordy for the card, as it also needs to fit three other abilities.
Moon: In D&D gives you 1d3 wishes, which in Magic are tied very much to the wish cycle from Torment. Maybe a slow wish could be something like "You may choose a card you own from outside the game and put it into your hand." I mean, technically better than almost all of the wishes, but the playable ones are either cheaper or instant speed, and give you the card immediately. The downside is that you can put an answer to Deck of Many Things in your sideboard and never have to worry about it's downside.
Star: In D&D increases one of the player's ability scores by 2. Well, like Comet, that's pretty hard to translate to MtG as well.
Sun: In D&D gives the player a truly epic amount of experience points, plus a "random" wondrous item (which can be anything from a simple folding boat to some of
the most outlandish items D&D has to offer). Yeah... This, again, is one of those cards that could be translated maybe (well, the wondrous item part at least), but makes for a wordy ability that doesn't really fit with three other abilities. I'ld make it something like a tweaked
Gamekeeper but for artifacts (and immediate instead of on dying). So, basically "Reveal cards from the top of your library until you reveal an artifact card. Put that card onto the battlefield, then shuffle your library."
Hmm... That looks shorter than I thought! Worth trying out maybe?
Throne: In D&D the player becomes a very persuasive leader figure, small castle included. Only problem is that that castle is currently occupied by monsters. In MtG I can't really see a short wording for this kind of effect, if it can even be properly translated. I feel Throne could get its own Magic card, and there still wouldn't be enough lines to do the card full justice.
Vizier: In D&D gives the player a truthful answer and the wisdom to interpret that answer to any one question. In Magic this could be represented perfectly by scrying for a bunch, looking at your opponent's hand, or
https://scryfall.com/card/arc/8/spin-into-myth]fatesealing(/c][/url]. None of those options are particularly appealing to me though, as scrying messes with shuffling back a card (i.e. you would always shuffle back first, so it dictates the order of abilities to a degree), and not knowing what your opponent is up to is one of the fun things in Magic.
And that's it folks! What do you think? What options would make for the funnest design?