vasik
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To give a hero 8 skills, you need to specify all 8 of them and the necessary perks in the hero's XDB file. They won't be visible in the skills window, but they will be displayed on the creatures, meaning there will be spirit, luck, attack, and defense, and basically everything. This works 100%, and I'll even attach a screenshot.
As for making 8 skills upgradeable, I don't know. Someone made 8 instances of HERO_SKILL_NONE; perhaps on a regular map, there will be an opportunity to upgrade them all when leveling up. I haven't tested this for sure; I just saw that someone did it that way.
Added 1 minute ago
Regarding "unrefusable" – I made 8 skills, but I reverted to 5. In reality, it's not that interesting, at least in PvP or pre-made battles that don't involve hero leveling.
There are a total of 12.
As for making 8 skills upgradeable, I don't know. Someone made 8 instances of HERO_SKILL_NONE; perhaps on a regular map, there will be an opportunity to upgrade them all when leveling up. I haven't tested this for sure; I just saw that someone did it that way.
Added 1 minute ago
Regarding "unrefusable" – I made 8 skills, but I reverted to 5. In reality, it's not that interesting, at least in PvP or pre-made battles that don't involve hero leveling.
There are a total of 12.