Mega Evolution Rules
1. Requirements
A Pokémon can Mega Evolve if all of the following critteria are met:
-The Pokémon is of a species that has a Mega form. Species in the same evolutionary family don't count; it has to be the exact correct species. (thus, Gardevoir and Gallade can Mega Evolve, only to their own respective Mega forms, but Ralts and Kirlia cannot Mega Evolve.)
-The Pokémon's Happiness is 3 or higher.
-The Pokémon's held item is the appropriate Mega Stone.
-All trainers participating in the battle have at least one Pokémon in their active squads that meets all three of the above critteria at the beginning of the battle.
-No Pokémon controlled by the same trainer has already Mega Evolved in this battle (i.e., each trainer can only Mega Evolve a Pokémon under their control once per battle).
Any of these requirements can be disregarded in a given unofficial battle, as long as all trainers participating have explicitly consented to such.
1.1 Exceptions
-Legendary or Mythical Pokémon who can Mega Evolve or undergo Primal Reversion (Mewtwo, Groudon, Kyogre, Latias, Latios, Rayquaza, Diancie) are exempt from the Happiness requirement. Primal Reversion otherwise follows all rules of Mega Evolution, with the appropriate Orb acting as the appropriate Mega Stone.
-Rayquaza is also exempt from the held item requirement and can Mega Evolve to Mega Rayquaza regardless of its held item. However, it can only Mega Evolve in an action immediately after it has used the move Dragon Ascent, and its held item vanishes from the current battle as soon as it has Mega Evolved.
1.2 Mega Stones
Mega Stones behave as any other item if they are being held by a Pokémon that cannot use them to Mega Evolve in the current circumstances (the base power of Fling with a Mega Stone is 30). However, if a Pokémon holding a given Mega Stone can use it to Mega Evolve or has already done so, slightly different rules apply. Mega Stones under those circumstances become strongly attached to their holder by a force that seems almost magnetic, and even practically becomes a part of their holder's own body once the Mega Evolution is triggered; as such, they cannot be removed, alienated or negated by any means -- not by even by the user's own actions.
2. Mega Evolving
Unlike the games, Mega Evolution is a regular command with neutral priority; it isn't done in the beginning of an action, and it's not possible to Mega Evolve and attack in the same action. The act of Mega Evolving must be triggered by command and consumes an action, as well as 5% energy. Additionally, as Mega Evolution follows the regular order of initiative in battle, all of its effects are only granted starting from the exact point in the battle when the Pokémon has Mega Evolved (as such: a Clear Body Metagross's Defense will not be lowered by a move used before it has Mega Evolved, even in the same action; conversely, a Mega form's Defense boosts, if any, will not lower the damage of physical moves used against it before it has Mega Evolved; furthermore, Magic Guard Alakazam will take damage from poisoning or burns at the end of an action that it has Mega Evolved in, unless it Traces an ability that prevents so, and an Early Bird Houndoom Mega Evolving while asleep will have its sleep duration normalized starting from the action of the Mega Evolution. Note also that non-passive boosts granted by Abilities, such as Attack gained by a HA Camerupt's Anger Point or lowered by a Gyarados's Intimidate, will remain even as these Pokémon change abilities upon Mega Evolving). The act of Mega Evolving is entirely unpreventable by external circumstances -- even where a move would normally be prevented by full paralysis, sleep, freezing, confusion failure, flinching, Taunt, etc., a Pokémon attempting to Mega Evolve will not be deterred, and any probability-based move preventions won't even roll against Mega Evolution.
2.1 Mega Form bonuses
While a Pokémon is in their Mega Form, some fixed changes will be made to its attacking and defending stats. These changes bypass the regular limits of boosts and deductions in stats -- so, for example, a Pokémon whose Mega Form gains 2+ Attack can still use Swords Dance three times, up to a walloping total of 8+ Attack (and, conversely, its Attack will never go lower than -4). The changes are specific to each species's own Mega Form. Additionally, some Pokémon will gain a different Ability, type, and/or base speed value when they Mega Evolve.
3. Regarding Signature Moves/Attributes
The ability to Mega Evolve cannot be given via Signature Move/Attribute to any Pokémon that doesn't already have it, and Signature Move/Attributes cannot be used to circumvent the requirements of Mega Evolution for a Pokémon who's capable of meeting them (eg, no Signature Move/Attribute can obviate the need for a Mega Stone to be used for Mega Evolution). However, Signature Moves/Attributes can have very similar effects to Mega Evolution, even openly analogous effects (as long as the approvers consider them sufficiently balanced), and can be given Mega Evolution flavor; this restriction is in place only in order to keep Signature Moves and Signature Attributes mechanically distinct from Mega Evolution proper.