Defense of the Ancients (commonly known as DotA) is a custom scenario for the real-time strategy game Warcraft III that became extremely popular. Valve would later buy the rights to DotA and create a sequel called Dota 2 and a series.
The objective of the scenario is for each team to destroy the opponents' Ancients, heavily guarded structures at opposing corners of the map. Players are a part of a team of five, using powerful units known as Heroes and assisted by AI-controlled fighters called Creeps.
This tag implicates dota_(series) (learn more).