Uninvited guests.

Moreover, even after defeating the opponent, artifacts are not awarded for some reason.

Just in case, I'll reiterate that artifacts will not drop from AI flagships. At the start of the game and with each regeneration, the AI hero receives a random set of artifacts to enhance their abilities. If these artifacts were to drop on the player's main hero after every victory, the game would become, in my opinion, uninteresting.

Added 1 minute ago
Unfortunately, I don't have time to work on the human and necromancer specializations. Suggestions from players are welcome.