DOTA was based on a StarCraft scenario, Aeon of Strife, and was released in 2003 by mapmaker Eul. It was later updated with a recipe system and Roshan by Guinsoo and renamed DOTA All-Stars, which became the most popular version of DOTA.
As punishment for killing a Shopkeeper and stealing the Aegis of the Immortal, the Gods cursed him to an eternity of being robbed and killed by Heroes in addition to being named after Guinsoo's bowling ball.
Valve's decision to develop DOTA 2 stemmed from the interest of several veteran Valve employees who played DOTA competitively. They spoke to the current developer of DOTA, IceFrog, offered him a tour of Valve, and eventually hired him to develop a sequel.