Eul is the original inventor of Defense of the Ancients. He developed his map inspired by Aeon of Strife (AoS), a Starcraft custom map developed by a mapmaker named Aeon64. Using the more powerful Warcraft III editor he was able to create a map similar to Aeon of Strife, but with 2 factions of players battling each other (instead the custom map for Starcraft featured only a team of players battling a computer controlled team).
DotA was born
He published his map in 2003, just some months before the release of The Frozen Throne, after a long beta testing between his online and offline friends. DotA wasn't the first or the only map to be inspired by AoS at the time, but it was widely regarded as the best "AoS-like" map and soon became the most map played on Reign of Chaos.  Eul then left the development of DotA to work to a sequel, called "Dota 2: Thirst for Gamma". Thirst for Gamma has never been a success and was still full of bugs, so the original DotA was still developed by other people, mainly friends of Eul at the forum TheWarCenter, then acting as the official forum for DotA.
Disappointed of the outcoming of Thirst for Gamma, Eul left the development of Warcraft III maps and for many years no one could know what he was doing. He resurfaced in 2012 when he was invited by Valve to assist at The International 2012. He also ceded to Valve every rights of the name "Defense of the Ancients" and supported them in court after the legal action took by Steve "Pendragon" Mescon (of Riot Games) and Blizzard over the copyrighting of the name. He said he loved how DotA has evolved from his first map.
In some forums Eul was registered as Eulogy. "Eul" is the contraction of "Eulogy".
Eul in The International 2012 audience.