What even is a mythic card?
Only one mythic card of each type exists and some of them completely change the gamestyle on the board.
How can/could I get these cards?
For now, you can only buy them from holders for a very big price and most of them won't sell it for any price. They got cards from raffles, which took place in 2019-2020.
(dates are when the cards were updated to the current version, as it's unknown when these cards were really released)
In total, there are seven different mythic cards:
1. Citadel of the Gods
This card probably changes the board the most. If you have it in your deck, at the start of every game, it does everything, which is written on it. You can summon any legendary creature from any god, it can be an 1 mana Deuteria, or it could be a 9 mana Echophon, Atlantean Hydra.
Luckily, there is a video showcase and even a gameplay showcase with this card:
showcase: 
gameplay: 
It was released on June 2nd, 2020.
2. Atlas Burden
This card is very good(especially if you use it when you have a lot of mana), but not absolutely gamechanging like the previous one. It is shuffled in your deck by another mythic creature, Atlas.
It was released on August 28th, 2019.
3. Promethean Flame
This is the card, which is shuffled in your deck by another mythic card, Prometheus
It was released on August 28th, 2019.
4. Hyperion
This one is the most well-known mythic card and that card was the reason I've started playing Gods Unchained. Hyperion is very gamechanging as it changes every card in your deck to 10/10 frontline with 1 mana cost.
Sadly, this amazing card doesn't have any showcases or gameplay videos.
It is one of the two mythic card which were sold. The price was 146.28 ETH which was roughly 60k USD back there. Now 146.28 ETH = 610k USD
It was released on August 28th, 2019.
5. Prometheus
This is a very strong card as it's 20 stats for only 6 mana, but you need to regen it or be very fast with it as it has burn 3 and gives your god the same amount of burn.
It shuffles 15 Promethean flames into each player's deck. I've showcased this card a bit earlier.
It was released on August 28th, 2019
6. Atlas
This card is quite close to the previous one, but it is worth 7 mana and shuffles different creatures. I've also talked about Atlas' Burdens a bit earlier. This card is quite gamechanging as Atlas' Burden alone is a very good card. It is the second mythic card which was sold - the price was 210 ETH, which was only 31k USD back then.
It was released on August 28th, 2019
7. Tethys
Tethys has the worst fate from all the mythic cards - it was given as a result of a raffle to a person who played a bit and left the game. To this date, it's still unknown who has this card right now. I also couldn't find the info about a Blue Whale creature.
These are all mythic cards in existence for now. Do you think we will get any more mythics? Write in comments. Thanks for coming to my TED talk😂.