HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

I want to know what Horn of the Abyss is and what it's all about.

To form your own opinion, it's best to try it yourself. You can download it, for example, here.
http://www.ex.ua/search?original_id=625&s=horn+of+the+abyss

I was put off by the old-fashioned split of the first games, which, in principle, is solvable, and by the lack of new ideas with a pirate race. I'm afraid it will remain as it is.

But in general, if tournaments start being played in the add-on, I will be very happy).