Great Gathering! Legendary! Mirage! Mega Evolution! (Japanese: 大集結！ 伝説！ 幻！ メガシンカ！) is the fourth set in the Pokémon Tretta Ultimate Z series, and the twenty-second and final set overall. It features 48 Tretta.
As its name suggests, the primary focus of this set are Legendary and Mythical Pokémon, as well as those that can Mega Evolve. A total of 19 Pokémon capable of Mega Evolution are included in Great Gathering! Legendary! Mirage! Mega Evolution!, making it the largest selection of any set. The featured Ultimate-class Tretta of this set is the Mythical New Species Pokémon Mew, which can use its Z Tretta ability in tandem with the moveTransform to change into 1 of 7 possible Pokémon forms that have previously featured as Ultimate-class Tretta. These include: ShinyMega Rayquaza with the move Dragon Ascent, Kyurem with Glaciate, Arceus with Judgment, HoopaConfined with Hyperspace Hole, Hoopa Unbound with Hyperspace Fury, Zygarde with Land's Wrath, or Mega Mewtwo with Psystrike. Mew will attack immediately after transformation and then revert back to its original appearance.
Hoopa from Hyperspace Fury! Hoopa Transformation is included in this set as the re-issued Ultimate-class Tretta, while Mewtwo is featured as high-class Pokémon for the second set in a row, this time on two Legend-class Tretta. Both Mewtwo have the same stats but have different Mega Evolutions and moves: Mega Mewtwo X can use Focus Blast, while Mega Mewtwo Y can use Psystrike. Among the Master-class Tretta in this set are Primal Groudon and Primal Kyogre, which have the Z Tretta ability. Each can summon the other if used alone in Battle Mode, but are guaranteed to summon Rayquaza if used together. Legendary beastsRaikou, Entei, and Suicune with the Z Tretta ability are once again included in this set, this time as Hyper-class Tretta. As with their initial counterparts from Birth! Z Tretta!!, each member of the trio can summon either of the other two members if used in Battle Mode, but will always summon Ho-Oh if all three are used together.
All regular Tretta in this set have a dark sapphire color. The locations available and overall seasonal theme in Search Mode remained completely unchanged from the previous set.
Challenge Battle included five new line-ups for players to earn a final set of five stamps on their Stamp Card. The difficulty ratings remained the same as the previous set, with Top Leaguer Tatsuya returning as one of the 5-star challengers. As Great Gathering! Legendary! Mirage! Mega Evolution! was the last Pokémon Tretta set, no Pokémon were available to add to player's Extra Attacker rosters.