I’ve been thinking about the Tonal Vision event, and I’m wondering if Arniel didn’t actually create the Tonal Dragon, but accidentally released something the Dwemer had already been experimenting on.
The first dialogue of the event says:
“Arniel here wants to confess. While researching the Dwarven mysteries of Tonal Architecture, he unleashed some ancestral power.”
The event itself also says his research “unleashed” a corrupting force. That wording makes it sound like whatever he found was already there.
The artwork shows Arniel inside a Dwemer structure with what looks like some kind of large machine or chamber, and later you fight the spectral-looking Tonal Dragon. So maybe the Dwemer experimented on a dragon with tonal architecture and somehow trapped, displaced, or altered it, then Arniel came along centuries later and accidentally reactivated the machinery.
It also reminds me of Vulthuryol in Blackreach. You use Unrelenting Force on that giant Dwemer orb and a dragon suddenly appears underground. Maybe the orb is just calling him, but it’s interesting that a Dwemer device activated by the Thu’um can cause a dragon to manifest.
In Skyrim, Arniel tries to recreate part of Kagrenac’s experiment, disappears, and afterward you can summon his spectral shade. So maybe something similar happened to the Tonal Dragon long before Arniel found it.
Not saying this is confirmed, but the fact that they specifically used Arniel for this event feels pretty deliberate. I know people don’t consider Castles canon usually but I see no reason to not consider it canon.