No, the Darkest Spell is Eclipsa's creation. Solaria never found a way to succesfully kill septarians.
Globgor told Eclipsa that Seth of Septarsis was leading the anti-Mewman movement, meaning that his army was the only real threat to Mewni.
Problem was septarians are inmortal.
In order to stop the war before abdicating (so Mewni would be safe after she was gone) Eclipsa created the first spell capable of killing a septarian: The Darkest One
But it had side effects (the corruption of magic) and decided to sace it for important individuals, most likely generals.
The spell was in fact so dangerous she decided to write in in low Mewman, so only the royal family could read it and use it in case of emergency.
It's all in the Book.