Legendary has 7 tiers of steadily increasing requirements and difficulty
Mythic has 3 tiers
1. The exact same as t7 legendary
2. A bit harder than t7 legendary but you can use anything in your roster that's 7*
3. A very difficult match using the legendary character
Replies
The mythic one is a little bit harder event with other "normal = no character" rewards. The mytic one is unlocked after you beat the legendary one.
Mythic has 3 tiers
1. The exact same as t7 legendary
2. A bit harder than t7 legendary but you can use anything in your roster that's 7*
3. A very difficult match using the legendary character