Completely natural.
Example: the hero started with 3 knowledge (15 mana), and during leveling up and buying artifacts, reached 10 knowledge, but still has 15 mana.
The next day (before the battle), he recovers 10*2 = 20 mana, according to the natural mechanic (2 per knowledge point), for a total of 35 mana (with a maximum of 50).
Visiting a well will be available, which fully restores mana to the maximum, but for an additional cost of 500 gold per mana point (if there is not enough money, it restores below the maximum). This simulates the fact that when playing on a map, in order to face the opponent with full mana, we have to spend time visiting wells, or save mana during exploration (and time is converted into gold according to the arena's ideology, so gold is given for logistics). Losing time = losing gold.
Mysticism: restores an additional 1 mana for each knowledge point, the same effect on the arena (natural).
Intelligence: increases the maximum mana by 40% (from 5 to 7 per knowledge point), the same effect on the arena (natural).
Dark Regeneration: restores 1 mana for each hero level before each battle, the same effect on the arena (natural). If a well was visited, there is nothing to restore (mana is already at maximum), and the regeneration will not work (but the native effect in battle remains).
Dark Ritual: reduces the cost of visiting a well by 2 times (1 mana costs 250 gold instead of 500). An analogue of DR for the map, where using the ritual partially spends time, but overall saves it (by setting the cost for the ritual to 50% of the turn, not 100%).
Secret Advantage (SA of the Dark Elves): gives +5 to spell power and +50 mana. Mana will be given immediately, not the next day (natural effect).
Thus, in the previous example with 35 mana out of 50, the hero will start with 15+50 = 65 mana. In fact, the ability gives a lot of mana, but disables mana regeneration, visiting a well, and makes buying knowledge pointless, since the mana limit is already exceeded.