Dark Dimension Requirements are different for each Difficulty.
Dark Dimension I requires you to have 5 Characters with 6-Yellow Stars to unlock. You can battle each node with at least 1 eligible character.
5 Gear Tier 13 Characters are required to start Dark Dimension II. Each node can be battled with at least 1 eligible character.
Dark Dimension III needs you to have 5 Characters at Gear Tier 14 to start. You can battle each node with at least 1 eligible character.
Dark Dimension IV can be unlocked by having 5 Characters at Gear Tier 15. You must use 4 eligible characters to start each encounter.
The newest difficulty, Dark Dimension V, is available for players with 5 Characters at Gear Tier 16. 5 eligible characters are required to launch each battle.
Note that Dark Dimension III, IV, and V nodes may only allow characters with specific Traits: Non-Legendary Global, Non-Legendary Cosmic, Non-Legendary City, and Legendary.
Dark Dimension I requires you to have 5 Characters with 6-Yellow Stars to unlock. You can battle each node with at least 1 eligible character.
5 Gear Tier 13 Characters are required to start Dark Dimension II. Each node can be battled with at least 1 eligible character.
Dark Dimension III needs you to have 5 Characters at Gear Tier 14 to start. You can battle each node with at least 1 eligible character.
Dark Dimension IV can be unlocked by having 5 Characters at Gear Tier 15. You must use 4 eligible characters to start each encounter.
The newest difficulty, Dark Dimension V, is available for players with 5 Characters at Gear Tier 16. 5 eligible characters are required to launch each battle.
Note that Dark Dimension III, IV, and V nodes may only allow characters with specific Traits: Non-Legendary Global, Non-Legendary Cosmic, Non-Legendary City, and Legendary.