The best way I can think of explaining this is with Teddy Roosevelt’s quote “speak softly and carry a big stick.” Someone who can wield a sword but chooses not to. A person that people respect, but know not to cross him/her because they’ll regret it. What would be the adjective to describe someone like this?
The closest one I know is iron fist in a velvet glove.
An iron hand/fist in a velvet glove: used to describe someone who seems to be gentle but is in fact forceful and determined — Cambridge English dictionary
I would call such a person -- one who would use power only if necessary -- judicious. From M-W:
judicious: having, exercising, or characterized by sound judgment
A judicious person would certainly be worthy of respect and, when necessary, fear.