Sounds like a ninja?

Hai guise.
May i ask you for a favor?

Could you translate this into romanized japanese:
"In the name of justice, you are now our prisoner."

The thing is, im going to role play as a ninja and i don't know any Japanese, but i want a cool line to drop at some point that at least sounds like Japanese.

Maybe you have some other suggestions?

Google translate suggests:
Seigi no na nioite, anata wa ima watashi-tachi no shūjindesu.

Sounds legit?
Maybe replace anata with omae or something to make it less polite?

正義のために、　お前は俺たちの奴隷だ！

Actual, authentic way:

正義にかわって貴様は我が囚人でござる
Seigi ni kawatte kisama wa waga shuujin de gozaru

>>4
or
正義の名の下に貴様は我が囚人でござる

>>5
ござるはやめろｗｗ