NHF – new heroes for Heroes V 3.1.

In the demon campaign, there's a mission where the fog of war closes. Maybe you can see something there?

This script prevents opening the fog of war in the standard way; it doesn't allow hiding it for the entire map.
It can't be configured for specific parts.

SetWarfogBehaviour(onLand, onSea);
Instead of onLand, write 0 so that the player cannot "remove" the fog of war by moving on land; otherwise, write 1.
Instead of onSea, write 0 so that the player cannot "remove" the fog of war by moving on water.

Added 14 minutes later
You can create a feature where, when used, it hides all open parts for opponents. But this is more suitable for the NTBF mod. For example, when building a building. But this is no longer "Veil of Darkness." And it's a one-time effect, unless otherwise configured.