Every story comes to an end eventually... and Mewlt's story is an especially arduous one. At the end of his journey, Mewlt and his friends confront the ultimate evil... and the final battle begin. No championship titles or silly trophies here ; the winner takes all, the loser pay the ultimate price.

Part 1:
Legion
Element: None (Neutral to all)
Polarity: Shadow (weak to Holy-ranked moves)
Ability: Boss Enemy (can't run from battle + instant death moves don't work)
HP: 857
Attack: 255
Defense: 255
Special attack: 255
Special defense: 255
Speed: 10

Cherlie stands before the final foe... The ultimate abomination, Legion, is made of all of the lost and wasted souls from past, present, and future forming an infinite mass that takes the form of a massive sphere. Legion attacks by forming mock-ups amrs that ends with a mishaped Legendary figure that then unleash a move. The Legion isn't truly that overwhelming in this shape despite its appearance and massive HP count. Although it has no real weakness, a steady barrage of attacks as well as careful healing will get a well-prepared party through without many problems... but of course, this is only a warm-up.

Part 2:
Legion Core
Element: None (Neutral to all)
Polarity: Holy (weak to Shadow-ranked moves)
Ability: Boss Enemy (can't run from battle + instant death moves don't work)
HP: 1395
Attack: 200
Defense: 200
Special attack: 400
Special defense: 350
Speed: 300

As the sphere became unstable, it collapsed and Mewlt found himself, along with his friends, floating over an infinitely stretching plain of lost and wasted souls, the core soul of Legion leaving the shell to fight up close and personal, high above its body. In this shape, Legion has weak physical abilities but impressive special abilities. Its special attacks are hard-hitting single hits while it shower the party with weak multi-hit physical attacks. This form is very powerful but it is vulnerable to physical attacks. A few good physical-centered Pokemon with high special defense will eventually outlast Legion Core.

Part 3:
Nihilum
Element: None (Neutral to all)
Polarity: Twilight (neutral to all)
Ability: Boss Enemy (can't run from battle + instant death moves don't work)
HP: 4759* (Nihilum has technically infinite HP but after receiving 4579 points of damage, the battle ends)
Attack: 475
Defense: 150
Special attack: 450
Special defense: 125
Speed: 25

As the Legion was finally shattered, the core soul stopped toying and revealed its true form. Nihilum, the original Darkness and Emptiness from which everything was born and to which everything return when they cease existing, revealed itself. Nihilum has infinite HP ; no matter how much powerful and well-prepared an attack is, its meter will never drain one bit. Nevetheless, after sufficient punishment, Nihilum will be "defeated". Nihilum takes massive damage from every attack but deal absolutely outrageous amounts of damage as well. Most of Nihilum's attacks are trademark moves and are lethal. Nihilum also can unleash Mewtwo's annoying Galaxy Force attack. Only the most prepared, high-levelled, fully outfitted party will overcome Nihilum.

Part 4:
Celebi (Nihilum(2))
Element: Grass/Psychic
Polarity: Shadow (weak to Holy-ranked moves)
Ability: Finale (can't run from battle + instant-death moves don't work + takes no damage unless attack strenght is equal/greater than max HP)
HP: 250000000* (require special means to be drained)
Attack: 475
Defense: 150
Special attack: 450
Special defense: 125
Speed: 25

As Emptiness itself, Nihilum could not be defeated. Yet, the mortal soul of Nihilum sensed the attacks and became progressively annoyed... and Celebi, merged with her true form, woke up to finish the job. With Celebi awakened, Nihilum gain a few new attacks... but two must be noted. Final Solution is the most widely used new move and deal either instant death or massive Shadow damage. As Nihilum telegraph this move by using taunting you the turn before unleashing it, you have the opportunity to switch in a Shadow/Demon tank to absorb the hit. The other move, Unborn Sin, will inflict death upon the currently fighting Pokemon regardless of HP and ability (it inflict from 45000(!) to 950000000(!!!) damage) on top of halving the HP of all Pokemon not on the battlefield. This move take three turns to prepare so make sure to prepare accordingly and heal after it was unleashed. Now... Nihilum cannot be defeated normally. The only way to actually defeat Nihilum is to unleash Unborn Sin with Mewlt. This require you to first get BOTH Mewlt and Cherlie into "Awake Mode". Do so by fighting a few rounds against Nihilum. Once this is done, have Cherlie use "Pray". This will gather the power. Finally, place Mewlt on the battlefield and repeatedly use Unborn Sin (you will have infinite PP and if you removed it as Mewlt's Awake-only move, it will replace your learned move). The first few hits will deal low damage (1-15 damage) but will deal exponentially greater damage. On the ninth hit, Celebi will take from 260000000 to 700000000 damage ; greater than her HP max. Congratulations! You saved the world!