So, I've heard some things about these "Mystery Cult" things, like the Path of Spite for Criamon, or Hermetic Theurgy or Hermetic Alchemy and Talismans. Could someone explain these things to me in a bit more detail? Like, how one gets into a Mystery. Also, there just seem to be so many possibilities and options for these, that I'm feeling overwhelmed by how many there are, and don't know how to begin to choose one.
Mystery Cult in essence means two things: One is the Mystery Cult Houses - Merinita, Bjornaer, Criamon, and Verditius. The other is non-House affiliated Mystery Cults.
The Mystery Cult Houses are a secretive bunch, where the Hermtically trained apprentice is inducted into the Mysteries of their House. They offer Initiations where the magi delve deeper into the Mysteries over time. usually this is a thing you can't gain acces to otherwise, so not regular Hermetic of Supernatural Virtues. What you can and should Initiate is dictated by the specific House. Although Verditius also have Confraternities, which are looser associations of magi within the Cult who Intiate less secret and unrestricted things, like Puissant Forms etc. The Mystery Cult Houses usaually don't have a rigid rank based hierarchy with s set order of Mysteries to intiate.
The Esoteric Mystery Cults are associations of magi across houses who Intiate Mysteries in accordance with their basic concept. They are usually based on some rank and soem set order of Mysteries. They could Initiate both esoteric amd restricted Mysteries (like Theurgy) or the regular Hermetic or even General Virtues (like the Cult of Mithras).
Getting into such a cult requires you to either know about it, find a member, and ask to join. Or if they are very secretive they will find you, if you seem to be the type to be interested and who is useful to them. typically it involves interacting with one or more members, do favors to prove one self and so on.
I don't recommend than a Mystery Cult House magus joins yet another cult, as it is increasingly difficult to run cult business and initiations unless the troupe is big on playing NPCs for each other for prolonged periods of time. it may even be difficult if there are two or more players playing Mystery Cult House magi, as they can never involve their sodales in their Intiations since they are secret.
An idea could be to base a saga on a common cult. And in fact this is what we're trying in our Rhine saga, because our last saga had too little common goals to work for.
And which cult to choose do you ask? Well, whatever you find interesting, is the answer I'm afraid.
House Bjornaer has some cool Mysteries, they all involve affecting yourself, with becoming/getting a better Heartbeast. But Bjornaer can be very good in play as their Mysteries are thinsg they can use for most anything they do.
House Criamon is not a favorite of mine. Previus editions had them be odd and enigmatic in most any way you wanted. Now they are very well-defined, philosophically odd, and if you like me don't like that box...I know they have some interesting Mysteries, but I've yet to see a Criamon in play here in 5th ed.
House Merinita are also more tightly defined in their faerie ways in this edition, but most of it seems to work quite well. They have 4 paths to follow, 3 faerie ones (which you really need to be very interested in to make us of) and 1 nature path (which is a favorite of mine)
House Verditius can become ridiculously good enchanters, but unfortunately I find them very, very introvert often see them stay in their lab to enchant or in the library to study. Pity, because they could have potential.