*SPOILERS FOR DARK WORLD SEASON OF PONY POV SERIES*
During the Dark World Saga's endless time loops, For the record, Nightmare Eclipse, likely intended to 'deal with' other villains in other world lines after she was 'done with' Discord and had created her 'golden world.' Of course, there was one villain who never cowered before her.
She also tried to give Tirek a 'brush off death' many, MANY TIMES, wanting him neatly and swiftly disposed of so she could get back to business, seeing him as not even a nuisance... or at least what she's SAID. In truth, Tirek was the one being Nightmare Eclipse feared.
I imagine in one encounter, Tirek ate Nightmare Eclipse's magic (like Discord, her own power worked against her, as she couldn't even comprehend her magic being stolen as being possible).
In another she sent the other Nightmare Gallery (all five of them), and he just ate their magic instead and used it force his way into the Nightmare Stables, requiring them later to rebuild half of it.