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Vis, anything can happen. If Heroes of Might and Magic III has at least some elements from Heroes of Might and Magic II, then Heroes of Might and Magic V has nothing from the old parts... (we're not counting units and gameplay, because Heroes of Might and Magic III is more similar in that regard).
I remember the story about Tarnum, you just wrote it a bit awkwardly.
Vis, the world is completely new, with a new history, etc., but for some reason, "old" heroes are known and remembered there...
P.S. In Heroes of Might and Magic IV, the Chaos faction can buy a Death Knight who has the Tactics skill, and you can level him up if you really need to.