Ultimate hunter
Arcanine landed in the middle of a dense forest and looked around. He roared loudly in order to scare all the Pokémon in the vicinity. Rex told him to do so since he didn't want any interruptions. Rex then told Arcanine to sit off to the side and summoned Weepinbell from its ball.
Weepinbell looked around and seeing no enemies he relaxed slightly and looked towards Rex before making his way over. Rex patted Weepinbell's head and told it to stand still while he repeated the same process he performed with Arcanine.
Weepinbell who got the explanation from Rex was ecstatic to evolve since it was incredibly hard to advance much further in its current form. Upon placing the 3 stones in a triangle shape around Weepinbell he stepped away and got the system to check that everything was ok. He didn't want any issues to arise.
As soon as the system gave the ok, Rex threw the fourth leaf stone towards Weepinbell. As soon as it touched its body the stone began to shine. The same scene repeated itself, green beams of light burst out of the stone and shot towards the three other stones that were lying there quietly. They had been glowing green brightly due to the excess plant energy in the surroundings and soon began to spin around Weepinbell.
The stone that was making contact with Weepinbell separated and then shot the 4th beam towards Weepinbell which started the evolution process. A blinding light burst forth, threatening to blind anyone present. Rex and Arcanine covered their eyes since they couldn't handle it.
[Alert! Chance of increasing Weepinbell's power have been detected if it's under sunny day boosted by the blue sun ability,] said the system hurriedly. Rex was stunned and looked towards Arcanine who was sitting there with his eyes covered.
Rex "What does it have to do with Arcanine?" he asked quickly.
[Weepinbell is absorbing the power of the sun right now, if it has access to more potent sunlight, it might be able to produce a massive change,] said the system.
Rex trusted the system; it had never once let him down or been wrong. Arcanine was an example.
Rex "Arcanine! Use sunny day!" he shouted. Arcanine who was sitting there quietly instantly stood up and sent a pulse of energy outwards. The energy travelled everywhere before amassing in the sky where the sun was located. The temperature began to rise instantly. It was reaching uninhabitable temperatures, the grass burnt up and wilted, the trees began to dry up and die it was like a wasteland in the surrounding 100 meters.
Rex who was next to him had to cover himself in Aura in order to survive under such heat. Rex marvelled at the sight before thinking that if this was still weaker than desolate land, what the hell would that be like. He soon snapped out of it.
Rex "How is it system, is that enough?" he asked getting a little anxious. He didn't want to miss such an opportunity.
[yes, Weepinbell is absorbing the sunlight as is going through a complete change.] it said to the relief of Rex.
As he observed the situation, Weepinbell who was submerged in the blinding light was going through a monstrous change. It didn't even look like a normal Victreebell at all.
Its body elongated and turned into the shape of a chilli pepper, the leaves turned thinner and longer, much more like blades. There was a leaf covering its mouth and attached to it was a brown vine that connected to a smaller mass that looked like Scizor's claw. (Source: https://www.deviantart.com/knightlight9/art/Mega-Victreebel-772311915)
As soon as the evolution was over the blinding bubble burst open and Victreebell could be seen. It was much more streamlined, and a lot more menacing.
Rex gaped at the sight. This couldn't be called the same Pokémon, one looked like a potato with leaves and the other looked like something taken out of a horror movie. The tail looked around as if sentient. It stared at Rex for a moment before looking away. Victreebell rushed towards Rex and rubbed its head against his leg.
Rex 'system show me his stats'
*ding
name: Victreebell the Hunter Pokémon
o ancient DNA status: Locked
o level: 45
o bond: 93%
o height: 2m
o hp: 200
o attack: 320
o defence: 190
o sp. atk: 310
o sp. def: 200
o speed: 200
o ability: Merciless, Chlorophyll
o moveset: vine whip, growth, Giga drain, sleep powder, poison powder, sunny day, synthesis, magical leaf, stun spore, solar beam, power whip (New), leaf tornado (New), leaf blade (New), toxic (New).
Rex was very satisfied with the new moves learned. Power whip was an amazing addition to Victreebell's arsenal. Rex turned his focus on the ability that appeared next to Chlorophyll. There was also a difference between the Pokedex name and the one the system gave.
Rex "What is this new ability system? And why is it 'the Hunter Pokémon' instead of 'the flycatcher Pokémon'?"
[Due to the evolution, the Pokémon has morphed into something new. It wouldn't be a stretch to say that this one is the next evolutionary step in the victreebell line. Host's Victreebell is the ultimate hunter. Sharing its sight and senses with the tail it has, it basically achieves full 360 vision. It can also attack independently and can cast moves independently. It has limited sentience though. You could say it is similar to Zweilous except better, as it skips through the trouble of deciding the main head. It is also better as it retains some of its sentience which allows for much more complex movements and attacks.
As for the ability, it is basically a scaling ability, the more damage Victreebell inflicts to its opponent the more attack or special attack it gets in return. This can be chosen by Victreebell freely.
After a quarter of the opponents health is taken away, Victreebell will receive 0.5 stages worth of power that can be assigned to either the attack stat or the special attack stat. For example, if Victreebell beat's Machoke, it will have gained 2 stages' worth of power. But, as soon as the next Pokémon appears, it will lose one out of the two that it gained previously. it is similar to moxie, but the effects are shown throughout the fight, not after.
There is also a hidden loophole. if the opponent switches out before the current pokemon has been taken down, Victreebell's accumulated stages won't be halved since no pokemon was knocked out.]
Rex was surprised. there was a similarity between Merciless and Moxie but the former was better since it scaled mid-fight. and if the opponent was not careful and switch out multiple times while Victreebell was still on the field, he would become a one-man army.