The Trickster of Demons (Mythic Trait; Recharges after a Short or Long Rest). If Marsh is reduced to 0 hit points, he doesn’t die or fall unconscious. Instead, His Illusion disappears and the real Marsh appears. He regains 375 health, and the initiative count is rerolled. Marsh has advantage on initiative and has a +8 instead of the +6 he normally has. Marsh gains access to his mythic traits and the Surrounded action.

Innate Spellcasting. Marsh's spellcasting ability is Intelligence (spell save DC 24, +16 to hit with spell attacks). Marsh can innately cast the following spells:
At Will: Major Illusion; Mage Hand; Chill Touch; Toll the Dead; Detect Magic
3/Day: Invisibility; Mental Prison; Counterspell; Teleport; Phantasmal Force
1/Day: Power Word Kill; Weird; Maze; Illusory Dragon

Legendary Resistance (3/Day). If Marsh fails a saving throw, he can choose to succeed instead.

Magic Resistance. Marsh has advantage on saving throws against spells and other magical effects.

Magic Weapons. Marsh’s weapon attacks are magical.

Multiattack. Marsh makes 2 Staff of the Abyss attacks.

Staff of the Abyss. Ranged Spell Attack: +15 to hit, reach 60ft., 2 targets. Hit: 28 (4d8 + 8) Necrotic damage. 

Dark Words. Marsh speaks dark words in abyssal, any creature with 30 feet that can hear him must make a DC 20 constitution and suffer 10d6 necrotic damage and be stunned till the end of your next turn on a failure, on a succession suffer half damage and ignore the stun.

Surrounded (Mythic Action Only). Summon up to 5 illusory duplicates in unoccupied spaces with 60ft., the illusions can be seen through with a DC 20 perception check, if you can't discern the real one you must attack at disadvantage against any of them. Clones have Marsh's AC and 1 health. When destroyed the explode in arcane energy dealing 1d10 force damage to any creature within 10ft. of it. Marsh can telepathically command them to move on his turn.

Marsh can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Marsh regains spent legendary actions at the start of his turn.

Casting (Costs 1 Action). Marsh can cast an at will spell from his list.

Illusions (Costs 2 actions). Marsh creates an illusory version of any creature he can see, it has all of their statistics and any class levels it may have. It acts on Marsh's Turn and is commanded to take actions by Marsh as a bonus action. The illusion only has 1 Health point, and is destroyed if hit.

If Marsh's mythic trait is active, he can use the options below as legendary actions.

Surrounded Dance (Costs 2 Actions). If Marsh has an of his clones from his surrounded action he and his clones can make one ranged spell attack against one creature each, if their are more clones then creatures the remaining clones do nothing. Marsh and his clones fire dark tendril like beams at one creature each, upon hit they suffer 5d8 + 8 necrotic damage.

Consume (Costs 3 Actions). Marsh grabs any creature within 5 feet of him and consumes the natural arcane energies from them, they must make a DC 23 Con save on failure: if the creature has spellcasting capabilities they must roll a d8 and lose the highest spell slot equal to or less then that number and Marsh regains one use of one of his 1/day spells, if no spell slots are available treat them as having no spellcasting capabilities, if they have no spellcasting capabilities then they suffer 5d10 necrotic damage and Marsh regains one use of one his 3/day spells. If they succeed nothing happens.