Is there a way to create a transparent texture? I mean, not completely transparent, of course.

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There's a problem here; transparency is handled very poorly in Heroes. That's why, for example, in the NVS mod, the seventh-level ghosts aren't transparent but rather glow. Predictably and understandably, only the material mode works, which displays either full transparency or full opacity (I hope I've explained it clearly). Semi-transparent textures usually glitch.

You might rightly point out that ghosts are semi-transparent, and everything is fine, and that's true, but the model is well-made. Usually, when you can see the body of the same creature through, say, a hand,
the game glitches and completely removes the hand, as if we only see the body – this is fixed by using different degrees of transparency for such details. I worked on this in 2018, and I never want to do it again.