St. Peter's Square Obelisk

In the center of St. Peter's Square in the Vatican you can see an Egyptian obelisk 40 meters high.

This obelisk was erected in the square around 1586 by order of Pope Sixtus V and has an eventful history.

At the very top is a strange ritual: an Egyptian pharaoh kneels before a creature sitting on a chair with a large round head, looking like an alien in a spacesuit.

Why would the Vatican and the Pope want this? What lies behind this obelisk? Who rules over kings, pharaohs, and popes?