Whoa, I gotta process the idea of Tash having originally being a man. I don't think that's ever crossed my mind before! It's a good thought indeed....

And if Tash (the idol in Tashbaan and the demon, and possibly before all this same demon worshipped by Jadis in Charn) looks a bit like certain assyrian portraits often now called Nimrods, since Nimrod was also originally a man, this doesn't make it worse.

The fact that Jadis mentioned temples makes me curious about what sort of religion(s) they had in Charn. Ideas, anyone?

Tash worship?

And then in Narnian world it was Jadis who initiated the human ancestor Tash-the-first-Tisroc in it?

That's a possibility...
I've always assumed that Tash was either a Narnian Satan-expy, and thus a fallen Star, or a Calormen-area nature-deity (similar to the river-gods in Narnia) who got too big for his britches. But I do enjoy hearing alternative explanations.

If he was similar to river gods in Narnia, he could even so be ancestor.

But in fact he does look like a demon having a relation to Apocalypse, from Last Battle, and that would fit Abaddon being his identity (not just counterpart in our world).

In that case, "Tash the ancestor" would be some man unhappy enough to have worshipped the demon and to have played his role.

There are many dangerous things in nature. I don't see it as impossible for a nature-god to have a dangerous, even a deathlike, appearance. 🙂

But it is an interesting alternative hypothesis that Tash of the Tisroc's line could be a man who was, in a manner of speaking, roleplaying the god. Possibly someone deeply disturbed, or possibly a priest who claimed to have been possessed by the god to become his mouthpiece?

Tash is clearly a demon, not just a nature god of the dangerous type.

I am not sure about "deeply disturbed", but I am sure about deeply ambitious. And such a man may well have been possessed by a demon.