@datagod,
> First, this will be a soldier's tattoo, so I want to make certain it's accurate.
Please read this.
> Second, some sources spell PLENA instead of PLENNIS.
I agree with
PLENA, Nominative singular feminine of the adjective
PLENUS, meaning "full" or "complete".
> Third, the soldier might like to revise the words to mean "the tyrant's
> fear is justified," or "the tyrant's fear is great and justified."
If he does, post the request here.
> Fourth, is there a difference in meaning when a V is used in place of a U?
No, in Latin they are the same letter. It was only later on (Late Middle
Ages or Rennaissance) that by convention V was used as a consonant and
U as a vowel. I generally keep to that convention.