Well, the withholding order that prevented his deportation to El Salvador. That, in turn, was issued pursuant to several US legal authorities.
And when Stephen Miller says
"Now some have said, well, but he had a thing called a 'withholding order.' A withholding order means you've been ordered deported, but an immigration judge is saying you cannot go back to a particular country. Here's the thing: if you are a member of a foreign terrorist organization, you cannot have a withholding order.""
He is just making things up. The withholding order is an order of the court. It can't be thrown out just because Miller gets a hard-on from torturing people. There is no "terrorist" exception to any of the binding treaties the US has signed, nor to this law:
(3) Restriction on removal to a country where alien's life or freedom would be threatened
(A) In general
Notwithstanding paragraphs (1) and (2), the Attorney General may not remove an alien to a country if the Attorney General decides that the alien's life or freedom would be threatened in that country because of the alien's race, religion, nationality, membership in a particular social group, or political opinion.
The only exception relevant here would be if "(iv) there are reasonable grounds to believe that the alien is a danger to the security of the United States" and that is clearly false. There are no reasonable grounds to believe he is a danger, which is why the withholding order was put in place.