Various mods by zahar0z

In other words, the concept results in an ability that is mostly useless, but in rare situations, it's almost overpowered, which, in my opinion, is a bad decision.

As far as I understand, you're talking about how the ability is implemented in the original game, right? Well, I wouldn't call it overpowered, nor a particularly rare occurrence, let alone a bad decision. And frankly, your statement is very superficial. You're forgetting that "it can only be used with this and that hero" and that the hero often has Fireball. And I also don't understand what makes it overpowered; it simply increases the damage of specific attacks in the future; it's not an ability in the style of "destroy everything in two turns." In short, it's an example of good situational mechanics that allows for interesting combinations. It doesn't hinder the specific golem at all, and it will be interesting to use it in conjunction with someone else or simply in the hero's army.

Added 8 minutes later
I also don't like the combination of "aura of vulnerability" and the mark. First, I don't understand why a fire unit would have vulnerability to fire. Second, it's not very proactive (it still needs a little more initiative reduced), and its vulnerability can easily be used against its own allies. Third, the fire mark is not applied very often, and in the case of this unit, only when it attacks an enemy in melee. In short, by the time your "ultimate buffer" reaches, attacks, and leaves a fire mark on someone after the third attempt, there will be nothing left of it.

Added 17 minutes later

Acidic blood would be much better if you want to have a poisonous ability. And in general, the balance of the boss depends largely on what I wrote earlier: whether this creature will appear randomly on the map or be encountered under certain conditions in a dungeon or something similar.

I like the idea of acidic blood. And I agree that the poison should be removed. And regarding where the boss will be, I've probably written about it five times already that it won't appear on the maps as a neutral creature (because then it wouldn't be a boss, but just an overly tough neutral). It will be a single guardian in the sunken Temple.