Tartarus is an icy inferno.

You can immediately tell that this person knows the history of the Heroes series.

This person has been playing Heroes for a good 12 years. It's a shame that after such a long time, they haven't bothered to learn the storyline. ;)

And one question - will it also replace some castle, or will it be added as a new one?

This is a difficult question. The "fathers" of Heroes of Might and Magic assure us that it is currently impossible to add new cities at all. On the other hand, there are people who claim that a new city can be implemented using scripts, replacing another one.

It is still too early to guess how Tartarus will be implemented in the game and whether it will be made at all, but for now, I can only say that the Cove will replace the Stronghold.