It's really cool in my head...Â

What is the idea of this card?
The idea was a card that's very good in your opening hand
but almost forces you to mulligan, works only once even if you mulligan into more copies, and is
completely worthless otherwise.Â
"Any time you could mulligan and Chancemonger is in your hand, you may reveal it and destroy target Chancemonger."
In other words, it's Legendary so you can't have two or more in play on your first turn. If you mulligan into another one, there's a fifty-fifty chance you'll mulligan a third time, because the dead card effectively leaves you with a five-card hand anyway, so at least you don't add insult to injury by shuffling it back into your deck. I'll broom it, though.
Isn't the first clause a waste of time? Once this is in play, the active player has priority and can play the sorcery speed effect.
As far as mulliganing into more copies, no, it's meaningful. As far as destroying an opponent's monger as I blithered about above, yep, that would be a waste of time alright. (kicks self in head) (again)
A very high casting cost (probably including the discarding of cards as well as mana) would probably be preferrable to no mana cost. Even FoW has a theoretical casting cost up in the top right corner. That would solve the problem of Mask and Aether Vial.
To prevent this from being put into play via other means, why not just make this an enchantment?
Fair enough. Letting you draw the cards by removing the specific counter that you can only put into play while mulliganing solves all the vial/ piper/ raise dead/ oath/ etc. problems, though.
Heh. I hit the post button for that and the board said, "Hey Chuckles, somebody else chimed in while you was scribblin'. Insofar as they're an odds-on favorite to be smarter than you, ya wanna have a look-see before you post?" Sweet!
Having said that why not make it a Sorcery and scrap the whole Sorcery speed clause. This is a one-off effect. Just give it an alternative CC similar to the new Guildpact cards and be done with it.
Until end of turn you may draw 3 cards if you have mulliganed. Play this ability only once.
Wow- that's deep. Does this prevent playing multiple copies on your first turn if you draw two in your opening hand/ mulligan into more/ etc? I'm sure we could live with Wishing them back into the game, Sins of the Past, and other remote contingencies...