My theory regarding the Narwhal.
Hello. I want to tack about this guy.
All-Devouring Narwhal - Visitor from the Far Side of the Sea of Stars
First thing first, as described by its lore-that thing is not from Teyvat but from another world. Like Traveler and other Descenders. So it is really difficult to tell - what it is, where it is from, its characteristics, how Skirk's master got it, etc. After all-it is not of this world
As far as I am aware, there was no such being in any MiHoYo games. But I saw something similar. Look.
This is a thing that appears in a hologram that MC of Honaki Star Rail watches after the first chapter of the story. We don't know what it is- but it looks like a giant cosmic whale. This wasn't a real thing but given that it was displayed on the train used by Universal Explores, they likely encountered it before. The differences in appearance can be due to different subspecies.
But for this, I also have a theory. Let's look at The Shadow- a mysterious being that looks like some Abyss monster that manifests out of nowhere.
I know quality suck- but it was the best that I could get.
We can assume that it is just that- an Abyss monster. But why - what connection they have. I can make two guesses.
- They try to corrupt it and turn int a weapon. Reasonable guess.
- This is no creature of Abyss at all.
I was surprised by the opening in his stomach-with a glowing sphere - on the left. Such opening and sphere are associated with the Antimatter legion of Star Rail.
See left part. The color pattern also matches.
So -what I imply is that not only Skirk's master can travel to another world (a super difficult feat requiring the blessing of Aeon, incredibly advanced tech or some other OP powers), but he also stole a battle beast of the Antimatter Legion to make a pet of it. And Legion is angry enough to send a relatively powerful member (looks different from ordinary members) to address the issue.
This can be a setup for a future collab event and definitely serve not only to demonstrate the power of Hexenzirkel's members (and that some may be Emanators), but can be a bridge to deeper tie Genshin to the wider Honkaiverse.
Thanks for your time.
Edit: After looking at description of its drop item, I changed my position a little. You can look here.https://www.reddit.com/r/childemains/s/mc5hoL12Pe
Essentially, it is said that Narwhal wants to "save" Teiwat by devouring it and remaking inside it. Which is REALLY similar to what Antimatter Legion believe in- that current universe is wrong due to all its flaws and we should help Nanook destroy it- leaving place for new one, better to be born.
So- I think that Narwhal is not a battle beast but a member. Its was generally sad about state of existence, eaten Legion member by accident. Said member that shared teachings of Legion with Narwhal- who embraced them. The two then traveled universe destroy worlds to make them reborn.