What is a law declaring an act illegal without a judicial trial?

The qustion should be clarified to state that it is about a law that declares a person guilty of commiting a crime, as opposed to declaring an act illegal, althought they can be read the same way. This is called a Bill of Attainder. Article 1 of the Constitution sets forth the powers of the Congress. Section 9, Clause 3 of that Article says that No bill of attainder shall be passed.