Skoll is the wolf who chases the Sun.
Hati is the wolf who chases the Moon.
At Ragnarokkr, they will devour the light from the sky, and their father Fenrir will break free.
There's lots more to the Norse myth, of course, more and plenty interesting and inspiring stuff at Fenrir's wiki page, but I didn't have anything in particular in mind while making this.
I just thought, Ragnarokkr (The gods' dimming/darkening) sounds perfect for Twi. And the sun and moon get eaten! It's just another one of those things waiting to befall Equestria...
Oh and, what was binding Fenrir in the first place? Well, there wasn't a chain strong enough to hold him, so the gods had something special made.
It was a ribbon; forged from six magical ingredients, a thin fetter that the wolf could not free himself from.
Elements of Harmony headcannon acquired~ ^w^