Dota 2 is its own engine, and has nothing to do with dota 1 which was a warcraft 3 map. How they even got the IP in the first place is... a pretty bad look for Valve.
What are you talking about? Firstly Dota 2 started in source 1 and was the first game ported to source 2. It has nothing to do with dota 1? It is made by multiple of the original developers of the warcraft 3 mod, your whole comment does not make a whole lot of sense.
It was made by the third (3rd) maintainer of Dota Allstars, the warcraft 3 mod. Eul was first, Guinsoo was #2. It had quite a large community attached. Valve hired Icefrog. Icefrog did not invent MOST of the characters, IP, nor mechanics, nor design the map. He DID balance a lot of the characters and interactions. I was an admin for Team Dota Allstars at the time and helped run the semiprivate leagues on battlenet and the dota-allstars forums.
So Valve hires Icefrog, makes an IP claim for the dota name, all the characters, and starts creating dota 2. It felt like a "oh, I'll have this". It didn't feel right at all at the time. Still doesn't. Nothing was ever said for the community that formed around creating and enjoying the game in the first place.
Both Eul and Icefrog assigned their copyright of to valve while getting employed at them, Guinsoo assinged them to Riot who then transferred the rights to Blizzard. And as far as I know Valve have only only done any kind of claim jointly with Blizzard as they are not the sole owners of the copyright.
They are though owner of the Dota 2 trademark.
Also a nitpick there have been more maintainers than those three they were the major ones for example the original developers of Dota Allstars Meian and Madcow. Guinsoo worked on the game for around 2 years until early 2005 and then Neichus took over and worked jointly with Icefrog who became the lead after Neichus left.
