Shukfir
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Maybe, but he only became Railag in the 5th installment.
Railag is a very interesting character because he has transformed many times in his life.
If you remember his demonic name (or rather, its English version - Agrael), you can easily see that Agrael is a High Elf name. Just like Dirael, Wingael, Vinrael, Arniel. The translation "Agrael" was probably chosen simply because "Railag" doesn't sound very pleasant.
I suspect that Agrael was originally a High Elf. Then he became a Dark Elf, Railag. Then, for some reason, he turned to Urgash and became Agrael again. However, he didn't become a full-fledged demon (remember: in Heroes 5, the Heart of the Griffin, which affected all demons, had no effect on him). Tieru cleansed Agrael of demonic corruption, and he transformed back into a Dark Elf, taking his former name - Railag.
As far as I know, Dark Elves originated from High Elves when they became obsessed with Chaos magic and thus turned to the dark side (like the Sith), as a result of which they were exiled to the dungeons; therefore, it would be more logical to introduce High (Forest) Elves first...
Well, the innocent have been made guilty. The transformation and exile of the Dark Elves are more the fault of the High Elves and humans than the Dark Elves themselves.
In reality, neither the High Elves nor the Dark Elves were ever particularly interested in Urgash's magic (except perhaps for some individual Clans and sects).
The story is somewhat different.
Initially, the High Kings in Erathia were chosen and appointed (there was, in a sense, a democracy).
But 36 years before the events of Heroes 6, High King Arniel decided that his son, Vaniel, would be his successor. Tuidhanna, the queen of a small kingdom on the border with humans, opposed this, and her lands seceded from Erathia.
24 years before the events of Heroes 6, Erathia and the Falcon Empire launched the Bitter Ash War (the very war in which Vyacheslav and Gerhart participated, and during which Gerhart's fiancée, Deirdre, sister of Kate, died). The fighting took place on the territory of the Kingdom of Tuidhanna. Humans and elves plundered these lands, killing innocent people. Tuidhanna asked Arniel for help, but was refused. What was the unfortunate queen to do? Her small kingdom was facing destruction. She was forced to ask for help from the Faceless Ones. But they had one condition: Tuidhanna and her people had to renounce Silanna and turn to Malassa, the Dragon of Darkness (by the way, Darkness and Chaos in Ashan are very different things). As a result of the ritual, the elves of Tuidhanna turned into Dark Elves. Thanks to the Faceless Ones, the Dark Elves drove out humans and High Elves from their kingdom.
16 years before the events of Heroes 6, Erathia, the Falcon Empire, and the Kingdom of Tuidhanna signed a peace treaty.
Two years after the events of Heroes 6 (beware, spoilers possible), demons set fire to Brittig, the sacred Elven tree. The new High King, Vaniel, decides that the Dark Elves are to blame for everything and offers them exile or death. Tuidhanna refuses to leave her lands, and she and her followers are killed. The remaining Dark Elves leave Erathia.
For 20 years, the Dark Elves search for a place to live, and only 22 years after the events of Heroes 6 do they settle in the dungeons of Igg-Shail, near the dwarves.
Yes, of course, it would be logical to introduce the High Elves into the game first (although there is a small chance that the add-on will introduce both Factions). But there is a possibility that the add-on will tell us about something from the history of the Dark Elves that does not concern the High Elves. For example, about their wanderings in search of a home. Or the war of the Faceless Ones with humans.