Gunpowder

Gunpowder's significant changes to Tamriel in 5E 001 made it the unofficial beginning of the Fifth Era (the official being the crowning of Emperor Kargas; he openly despised the Thalmor and began a campaign against them, aided later by guns)

Inventor
A Dunmer alchemist named Revul Alean accidentally created the explosive when experimenting with iron, two unknown powders and charcoal. After recording his discovery and beginning to sell it to merchants, he was assassinated, and all his writings taken to Imperials.