It was Harry Potter's first year at Hogwarts school of witchcraft and wizardry. He was not yet aware of the ghost that haunted the second floor girls bathroom. A ghost by the name of Moaning Myrtle who was as usual was miserable. Also usual was that the squib caretaker of the school Argus Filtch had gotten completely fed up with trying to keep the school clean.
What was unusual this year was that Filtch had decided to try a new method of keeping the school clean, muggle cleaning products. So one morning with Dumbledore's permission he had gone out a purchased a load of cleaning products. So many cleaning products in fact that the house elves had been having trouble finding places to store all of them. This led them to storing a large number of the products in a little used second floor bathroom. A towering pyramid of bleach next to the sinks in the bathroom.
One day when she was feeling exceedingly miserable because Peeves had been by to torment her Myrtle decided to have a good cry that flooded the bathroom and much of the second floor. This was also usual. What was unusual was that her crying and the wave of water that flooded the bathroom knocked over the pyramid of bleach, all of whom's caps had been loosened by Peeves.
So when the bleach was knocked over the contents of each bottle spilled out to mix with the water. The gas that it created didn't bother Myrtle because she was already dead and since most students avoided the corridor no one else noticed. Now this gas as it happens is heavier then air so it sinks down to the lowest place it can.
Thus the gas flowed beneath the sinks and straight down into the hidden Chamber of Secrets below it. There was so much gas in fact that for days the air in the chamber was completely replaced by poisonous gas, gas that the sleeping basilisk breathed in. Even for a powerful creature like the king of serpents it required air to breath and so with no-one being the wiser it suffocated to death on the fumes.
After that year and judging that the muggle cleaning products hadn't been as effective as he thought they would be Filtch got the house elves to get rid of all of the products. All of which would end up in a pile in the Room of Requirements, conveniently right next to a certain lost diadem.
In Harry's second year at Hogwarts, one Ginny Weasely possessed by the spirit of Tom Riddle went and opened the Chamber of Secrets to unleash the beast inside. However as the basilisk was dead Ms Noris escaped to warn Filtch and Ginny was captured before Sir Nearly-Headless Nicks deathday party was over.
It didn't take long for Dumbledore to figure out what the diary was and what Riddle had been trying to do. Going down into the chamber he recovered a basilisk fang and much of the creatures venom which he used to destroy the diary. Meanwhile Harry had an excellent year at Hogwarts free of any disasters or trouble easily helping Gryffindor to win both the Quidditch cup and the house cup, for the second year running in the house cups case.
Unknown to anyone else the chemicals in the Room of Requirements combined with the ambient magic in the room, ate through their containers and soon created a toxic magical gas that was drawn to strong magical objects. Like the nearby diadem. The gas flowed into the powerful artifact consuming its magical power and seeping across the soul link that connected all of the horcruxes. Eventually the gas had consumed all of the power within the horcruxes destroying them. Harry waking with a horrible killer headache one night that summer that soon passed. Thus Voldemort was defeated by the power he knew not, the power of muggle cleaning products and the fact that they should never be mixed.