In the following sentence:
La grand guerre ne cessera que mort ne soit vengée.
A direct translation is "The great war will not stop until the death is revenged". However, I do not understand the role of the second ne since it would mean "...until the death is not revenged".
Nostradamus verses isn't an easy path to to learn French.
The actual text doesn't contain what you wrote but:
La grande peste de cité maritime
Ne cessera que mort ne soit vengée
Du juste sang, par pris damne sans crime
De la grand dame par feincte n'outragée.
The style is obscure, even by 16th century standards but DeepL gives this translation:
The great plague of the maritime city
Will not cease until death is avenged
Of the just blood, by taken damned without crime
Of the great lady by feint not outraged.
The ne your question is about is what is called a ne explétif.
