In 450, the "main" and most obvious mystery cult is the Eleusinian mysteries. The middle of the 5th century is fairly early for the prevalence of mystery cults, and they definitely pick up steam and variety into the Hellenistic and then Roman period. In 450, other than Eleusis, there were probably options for the Dionysiac mysteries and the Orphic mysteries, but our evidence for them is pretty poor. We know a lot about Eleusis (relatively) because the bulk of our available evidence tends to be Athenian-flavored.

Pushing on down a few centuries or more, the options expand greatly: there were mysteries at Samothrace related to sailors and drowning at sea. I know the cult existed but I know basically nothing about it myself. There were "foreign" imports into the Greco-Roman sphere, like that of Isis or Serapis from Egypt, or the "Great Mother"/Cyble cult from the Near East, the latter of which the Romans physically imported (by dragging her sacred boulder across the world to install in the capital) and made their own. The cult of Attis, also brought to Rome late, was related to Cybele. Rome also pulled from relative obscurity the cult of Mithras, which became wildly popular in the Imperial period and was rivaled only by one other mystery cult, also a late-comer, the elephant in the room: Christianity.

There was also the weird cult of Sabazios, a kind of Thracian horse-hero who reminds me a lot of Conan the Barbarian.

As you can imagine, there is a lot of pseudo-academic mystic crap out there pertaining to this topic. It can be hard to find legitimate sources. Here are a few:

Walter Burkert, Ancient Mystery Cults (Harvard 1987).

Bowden, Mystery Cults in the Ancient World (Thames and Hudson 2010). NB: I have not seen this myself and it seems a bit like a mass-market book, but it could be good.

Giovanni Casadio and Patricia A. Johnston, ed., Mystic Cults in Magna Graecia (Austin: UT Press 2009).

Cosmopoulos, ed., Greek mysteries : the archaeology of ancient Greek secret cults (Routledge 2003).