Heroes 5.5 WarGame Edition - Tactical Edition.

I really hope it will be possible to implement it in this way. The developer said it might be possible.
Similarly, the dark ritual will also cost only 25% of a turn.

Is it possible to change the sworn enemy without a forced connection to a castle?

Added 6 minutes later
The upcoming mod completely removes late-game units.

Added 5 minutes later

Hmm, perhaps it would be easier to make the amulet "disappear" when the ultimate ability is acquired (i.e., destroy the amulet), if it's possible to preserve the racial bonus at level 4 using scripts.

It will be possible.
But linking the ultimate ability to an artifact, in my opinion, is not a good idea.

Added 7 minutes later
I still think the perk system 2-(1)-2-(2)-4 is ideal.
The numbers in parentheses indicate connections.
If we choose one line, initially only 1 primary perk of that school will be offered instead of two, which immediately increases the probability. Considering that 4 perks of level 3 are stronger than the previous ones.