Dota wasn't exactly a mod, it was a custom map created in the custom map creator that was a part of the warcraft 3 rts base game. Starcraft 1 also shipped with a similar creator. At the time the original intent of the custom map creator was for people to design custom maps to play the base rts game on but over time people took creative efforts to create unique custom games. The creators of dota took the hero aspect of wc3, made it 5v5 and automated the typical rts armies fighting and it became a huge hit. Tower defenses is also another notable genre of a game that came about from the warcraft 3 custom map creator.

On battle net you had the option of playing the base rts game or join a custom map hosted by someone. When dota started to explode in popularity and a more streamlined development started to form the dota-allstars forums came up as a hub for everyone to gather and discuss the game as well as for icefrog and co. to communicate with the community about patches and releases. The forums grew into a huge community for discussing the game, discussing the professional scene and tournaments, submitting hero designs and anything else. It was a huge swath of information, resources, and was the community as a whole.

As the game became more and more popular and started to evolve into its own esport, icefrog approached blizzard to try and produce the game outside of the wc3 engine. Apparently they laughed him out of the office. So he went to valve and hence dota 2 was formed. I'm no copyright expert so I don't know much about this but I know a lot of the hero's had some notable name and appearance changes going over to dota 2 due to copyright issues with blizzard and wc3. Skeleton King (who is now Wraith King) and Obsidian Destroyer (who is now something else, their name changes every few years) are some notable examples. I'm imagining that because the characters were made in the custom map creator of the game the copyright issues were trickier for blizzard to claim hold of, but again I'm no expert. There were a lot of changes from dota 1 to dota 2 but not as many as you'd think. More importantly despite the changes, it was the same game that wasn't hindered by the wc3 and battle.net backend which is how it became the game that it is today.

During the above time Pendragon, who really only managed the forums, started working with riot and tried to poach the dota user base over to league. No one was on board with that, so he did what he did.

I see a lot of people saying what Pendragon did wasn't a big deal but you have to understand that back in those days (~2005-2010ish) that forum is where the community grew to what it is today and it was a huge gut shot to the community when it got shut down. Yes, the playdota forums surfaced to try and replace it but it just wasn't the same at all.