Celestia - Daybreaker
Luna - Nightmare Moon
Twilight - ???

6982320

--Sweetie Belle

Eternal Twilight.

6982346
I like this one!

Um... let's see... Eventide?

6982320
Midnight Star

In story I loved but has long stopped updating, she was called North Star.

6982320
Morning Star, because it has a clever dual meaning...

A) The astrological event that happens during the morning Twilight certain times of the year.

B) The bad ass medieval weapon.

6982442
My Choices: Twisted Tales Through Time? So sad that’s no longer updating...

Anyway, I like Midnight Sparkle.

6982500
Ooh, better: Mourning Star

Shadow Sorcery

... how about just call her what she is?

Apocalypse...

6982320

How about Sunset Shimmer?

Dusklight Eternal?

Evenings curse

6982320
Afterlight?

Interestingly, "afterlight" has two meanings:

Noun

• the light visible in the sky after sunset; afterglow.
• a view of past events; retrospect.

Also in Equestria Girls, SciTwi's dark side was Midnight Sparkle, so perhaps that's canonical?

6982334
No Sweetie, that's Twilight Nightmare.

Twilight of the Gods.

Enmity

6982320
Gloombringer, Red Dawn, Duskholder, Nightfall, maybe add the demonic element with Night Fell, or if she mindwipes the Royal Guard as her first act: Knightfall.

Terminator

Although due to cultural contamination, no-one would ever give this the time of day.