This is my idea of celebrating Halloween, a muscle growth and possession fanart featuring Axol inspired by Dragon Ball Super episode 74. Not exactly my first case of Watagash, it goes to Lonefox's case, followed by another.

Story:
In the US, many people were said to be preparing for Halloween night, as in buying candies and setting up Halloween decorations. The same thing went to the Mushroom Kingdom. As Mario and his friends dressed up costumes as trick-or-treaters, Axol came to the castle late, as he had a recent anime conference a while ago.

Mario and Bob didn't take kindly to Axol joining the fun late and mocked him for it, only for others like Fishy Boopkins and Meggy to come into Axol's defense. The annoyed Garo spun with his blades against them, and then made a diss rap about how busy the axolotl was to the point of missing the best holidays every year. This prompted Saiko to smack Bob away with her huge hammer, and Tari and Meggy to comfort Axol. Mario became more annoyed with this outcome, but with glares from Saiko and Belle and with the rest of their friends going out to get candy, he lowered his head, yelled "Mamaf**ker", and left the castle in defeat. Bob recovered from Saiko's attack and joined Fishy and JubJub Boopkins, followed by the girls who remained, except Princess Peach who left the castle few hours earlier. The axolotl was the last to leave, with Toad coming into the hall and going to his room in celebration.

The axolotl was left mixed with embarrassment and anger over Mario and Bob's reactions. As walking through the suburbs, Axol happened to be literally behind most of his friends, who were not aware they were walking far away from him. Overwhelmed with emotions, he stopped by a long brick wall to relieve his frustrations, he noticed something on the road, crawling towards him. A small being came out of the shadows as a pink creature with many tentacles around it with a puffball hanging from it. Axol recognized it to be Watagash, a parasite that empowered its host based on how evil they were, but its appearance caused him to wonder why it came out of nowhere, as it never existed in SMG4's world. He suddenly remembered that Watagash was requested by Francis the nerdy chameleon, back in a day before they went on a plan to lift up an Anime ban on Mushroom Kingdom. That request was assumed to fail when the parasite was so small that it flew out of their sight.

Back in the same Halloween night, Watagash jumped and landed on Axol's left palm, much to the latter's confusion. It telepathically talked to the axolotl that it had been looking for any suitable host since its arrival to SMG4's world, barring Francis as even Watagash found him to be clearly pathetic. It noted some potential in its creator/summoner and wished to enhance him, but Axol insisted he was not evil. Watagash confirmed his statement, but added to him that darkness in one's heart was somewhat synonymous with any strong negative emotion, and that Watagash could sense Axol's feelings of anger and humiliation. Axol admittedly confirmed, but wished not to bring out his anger onto his friends, not even onto Mario and Bob. Watagash remained on Axol's palm for few seconds, not responding until he told Axol, that unless he managed to calm down, his resistance would never stop the parasite from possessing it. Much to Axol's shock and horror, the parasite pounced onto his chest, with its puff ball shoving into him.

Axol was left with pain for many seconds to come, as Watagash's powers came to effect. Muscles developed around his body, hard abs were formed, eyes turned glowing red, and then pain stopped. Afterwards, Axol spat out angry grunts and attempted to remove the observant Watagash by force, for 1 minute. Axol called the parasite out for possessing a good-hearted host just because of his negative emotions and demanded it to leave him alone, but Watagash was not impressed with Axol's new body not being intimidating enough. When asked why it had to be the serious case for the parasite, Watagash asserted that Halloween was happening today, which would give the parasite the advantage of causing chaos without early interference from the military.

Before he could respond, Axol entered another painful phase with Watagash injecting deeper into the axolotl's heart, forcing height growth, enlarging muscles, outgrowing most of his clothes. That phase ended with a wrathful roar from Axol. Learning from its past encounters, especially those with Gohan, it had already upgraded its effects, way before being summoned by Axol. The axolotl thought that should someone barge in and free him from the parasite, Watagash sensed his thoughts and revealed that he developed its effects where to the point that its host became corrupted with evil and remained in the current form in the way it wanted. The parasite then manipulated his mind, bending Axol to accept his new form and to embrace his new dark path. His growling teeth turned to widely mad grin, and went off to an evil laugh, so loud that his friends could hear him just several blocks away. Watagash was satisfied with the result, it could have left the axolotl alone, now that he was corrupted into a permanently muscular brute, but Watagash decided to remain onto its summoner as it was genuinely happy to be with him and it would be happier to see him destroy everything in its path. Only then would his friends come and face him with some way to turn him back to normal for good.

Axol the Axolotl owned by SMG4
Watagash owned by Dragon Ball