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Based off a response by Zasko I looked into mythic dungeons more. Below is a link to an 8 minute video about what mythic dungeons are all about.
TLDW(Too Long Didn't Watch): It will be similar to Dialbo 3's Greater Rifts.
You will have keystones to specific dungeons.
Each Mythic will have a timer. You get loot if you don't beat the timer. The keystone is upgraded if you beat the timer.
Each level of keystone increases the health and damage of the trash/bosses in the dungeon
Affixes: Additional modifiers on your dungeon run based on keystone level.
Level 1-2: Keystones have no affixes.
Level 3-5: One Affix
Level 6+: Two Affixes
Level 3+ Affix list
Tyrannical: Bosses have significantly more health & damage.
Bolstering: Trash mobs buff their allies when they die.
Raging: Trash mobs do double damage at low health.
Teeming: The dungeon has extra trash.
Level 6+ Affix List
Skittish: Enemy pay far less attention to threat generated by tanks or tanks generate far less hate.
Necrotic: Enemy melee applies a stacking dot & healing debuff.
Volcanic: Eruptions of fire appear at players' feet.
Decay: The group takes constant damage over time.