Defense of the Ancient's path from an ultra-popular Warcraft III mod to the core of Valve's ultra-popular MOBA, Dota 2 is a long and confusing saga. Now, two developers working on their own mobile versions of Dota are arguing in court that Valve didn't actually acquire a legitimate, enforceable copyright that encompasses the entirety of Dota's development history. This question now seems set to go to a jury.
Federal Judge Charles Breyer of the Northern District of California lays out an excellent summary of Dota's development history in a recent ruling denying summary judgement for Heroes Charge developer uCool. Breyer explains how Dota started life in 2002 as a Warcraft III mod made by Eul (real name: Kyle Sommer), who created the mod on his own and thus holds the rights to the "setting, heroes, rules, and name."
From there, in 2003, modders started to take "the best, most enjoyable characters from all the other version of Dota" and split off a new version of the mod called Dota All-Stars. A modder going by Guinsoo (real name Stephen Feak) soon took over this branch of the project, accepting and coalescing contributions from the modding community (unlike Eul, who largely worked alone).