In the world of Pokémon, main protagonists, Ash Ketchum and his partner Pikachu are far from immortal. Though they appear to be ageless and time seems irrelevant, the two have died more than once. Ash's most famous death came when he ran between the crossfire of Mew and Mewtwo in the Pokémon: The First Movie, which turned the trainer to literal stone -- only for him to be restored by the tears of Pokémon and clones alike.
However, that wasn't the first time Ash perished. He also died in the Pokémon anime near the start of his Indigo Adventures. In the Lavender Town, Ash and Pikachu were murdered in a series of ghostly events that culminated on Ash and Pikachu becoming spirits and pulling spirited pranks on Misty that, from her perspective, came straight out of a horror movie.
Terror Before the Tower
In Pokémon Season 1 Episode 22, "The Tower of Terror," Ash travels to Lavender Town to capture a ghost Pokémon. It truly is a matter of life and death for Ash already, since, as of the episode before, Ash had been almost imprisoned forever in a doll house by the psychic gym leader Sabrina of Saffron City. Sabrina's almost insurmountable power led to Ash losing brutally to her Kadabra, and, as punishment, the gym leader and her sentient doll -- representative of the "innocent" part of her mind -- used their powers to shrink Ash and company to play as dolls.
Ash needs to beat Sabrina to accomplish his dreams, but he'll never be able to challenge the Pokémon League if he doesn't get Sabrina's Marsh Badge. However, an old man with psychic powers -- Sabrina's father -- gives Ash some advice: go to Lavender Town and capture a ghost Pokémon. While Misty and Brock are too scared of ghosts to travel to the Pokémon Tower, where most of the ghost Pokémon hang out, Ash fearlessly walks in, dragging Pikachu along with him.
Drop a Chandelier On Him
Ash almost immediately encounters a trio of ghosts: Gastly, Haunter and Gengar. Despite Ash's best efforts, he can't hurt any of the ghosts. But his efforts to capture one of them are halted abruptly when a chandelier falls on both him and Pikachu, crushing them. While it first appears as though they've been knocked out, Haunter extracts their souls from their bodies and leaves their lifeless bodies behind.
However, after a moment of existential panic about dying, Ash and Pikachu really start to enjoy being weightless poltergeists who can soar through the sky and haunt the living. This is most notable when Ash picks Misty up into the air only to drop her into Brock's arms. This immediately alerts Misty to the fact something is wrong with Ash -- she seems to be aware it's Ash lifting her up on some level -- which leads to the two rushing to Ash and Pikachu and pulling them out from under the wreckage.
At the same time, Ash and Pikachu are brought to the ghost Pokémon's room, where they learn the ghosts just enjoy fun games and playing pranks. While they want Ash and Pikachu to goof off with them for the rest of eternity, Ash explains to them his dreams of being a Pokémon Master, and they help them return to their bodies, despite their disappointment. When traveling back to Saffron City to re-challenge Sabrina later on, Ash finds that Haunter has followed him back. Eventually, Haunter's pranks prove essential to him earning a Marsh Badge.
Morbid Reality
While the two of them are only dead for a few moments, it's clear that Ash and Pikachu did in fact die, and that Pokémon were responsible for it. While very few characters actually stay dead in Pokémon -- though Latios from Pokémon Heroes and Hunter J very well are dead for good -- it's clear that Ash and Pikachu have died way more times than anyone would like to admit.
And if Ash and Pikachu had stayed dead, they may very well have turned into ghost Pokémon themselves. Many Pokémon that would be introduced in later generations are actually the spirits of the departed, such as Phantump and Yamask. Phantump's Pokédex entry from Pokémon Sword reads "After a lost child perished in the forest, their spirit possessed a tree stump, causing the spirit's rebirth as this Pokémon." Yamask's Pokédex entry from Pokémon Black says "Each of them carries a mask that used to be its face when it was human. Sometimes they look at it and cry." It's highly possible that, should Ash have remained a floating spirit, he might have become one of these Pokémon. This would have given his quest to become a Pokémon Master a grim irony, as he would become a Pokémon himself, albeit a sad, lonely one.