It's a crown.

Furthermore, we declare, we proclaim, we define that it is absolutely necessary for salvation that every human creature be subject to the Roman Pontiff. Unam Sanctam, 1302

That’s what the Pope said about himself in 1302. Here’s what he says in 1962:

In this Church of Christ the Roman pontiff, as the successor of Peter, to whom Christ entrusted the feeding of His sheep and lambs, enjoys supreme, full, immediate, and universal authority over the care of souls by divine institution. Vatican II, Preface, 2.

The Lutherans say that the pope is the antichrist. This makes sense when you start to read what the pope says about himself.

Here is the documentation: