yes, by the way, when I say "blade," I don't mean a killing blade, but the Vorpal Sword – Cave Lords, Demon City, level 6 – in addition to the attack, it removes 1 creature.

BUT – its alternative, the sword, deals 2 HP damage per enemy – that's a really cool blade. And the Vorpal Sword is just an additional nice bonus.

There is an ability called Crystal Skin, which grants immunity to the Vorpal Sword and Deadly Grasp, so it won't kill any king after all

Here is the king himself – see the screenshot. He has 28 attack and 18 defense from the hero (6 speed and 16 initiative are his own, the rest is from artifacts), everything else is his own. The arrow shows what will prevent Harbingers or Cave Lords from killing him.

So, firstly, you greatly overestimate this ability, and secondly, no one knows how to technically make it happen