#(but)

Or: I can make anything about Terra if you just give me a second

Fenrir is unique in another way - it's one of only two Eidolons to have a secondary ability (with the other being, of course, Carbuncle, whose abilities change if you have different gemstones equipped). Fenrir's secondary ability is Millennial Decay: a wind-elemental ability, one of the few in a game where a lot of things are weak to wind (he's such a good puppy).

✨So here's the thing about Millennial Decay✨

Fenrir only uses Millennial Decay if you have a special accessory equipped: the perfume Maiden Prayer.

And the thing about the Maiden Prayer is. It's only found in two locations (specifically Ipsen's Castle and a mountain crack near Oeilvert). And both of those locations have ties to Terra. And that's interesting, because -

The red moon is always a direct reference to Terra. It's not a coincidence that the perfume only shows up in Terran locations; Millennial Decay's animation specifically invokes it.

Why does a Gaian Eidolon respond to what appears to be a Terran perfume? Why is it made stronger by it?

It's also interesting to me that only by using the Maiden Prayer does Fenrir... use its own element. Fenrir's standard attack is earth-elemental, but Fenrir itself is wind-elemental.