basically, Mythical Jokers are sorta like my take on a joker rarity stronger than legendary (with cryptid installed they're going to be rebalanced to be on par with exotic jokers, otherwise they will be stronger than legendaries, but still balanced to vanilla (somewhat)).
To obtain them, you will have to find the respective joker's shards, which will each have a 1% chance to replace a joker in shops and booster packs.
Although some of the mythical jokers will have some other, more unique methods of obtaining.
I will, also be including a secondary way to obtain a mythical joker, but this method will be a bit harder. A new Spectral Card called "The Vessel", which is essentially The Soul, but for Reverse Tarot cards. "The Vessel" is also much rarer than "The Soul", so it is actually a little bit easier to just find the shards.