^Of course they'll be older than what is written. I just mean since there is no time in those kind of myths and legends, that it's possible that there were others before that one in particular.
Hell, they could all have been made at the exact same time all around the world.
*Each culture gets its first demon sword thinger*
Shall read through those soon.
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Okay now for my crack theory....
Some of it is kinda more probability, but other pieces are more cracky.
So.
Mephisto.
Bear with me and the factual info

This is going to be messy...Sorry if I forget to include points.
Medieval times: 5th century to the 16th century.
Vatican was built before then, and Rome became Christian before then it seems.
So. 200-ish years before the medieval ages hit.
But, of course, we can be fairly certain Mephisto has been in Assiah for an insanely amount of time - and I believe he's been with True Cross for awhile - I'm thinking the beginning.
Isis brought up that in some cases Mephisto is said to have worked under Lucifer. Maybe he was mistaken for Satan, but I thought...
[Welsh Mythos]
What if one's Gwydion and the other Gilfaethwy? Gwydion is a trickster god, and Gilfaethwy is his brother who also would play tricks. (I'm not sure who is older and who is younger)
I'm thinking Gwydion is Mephisto, since Gwydion is ''a cunning enchanter and patron of the arts and education''.
They do have a whole ''hind and stag incident'' but I'm leaving that out....Why do I bring this up? Maybe because at this point in time Mephisto and Lucifer were closer, and both enjoyed playing around in Assiah. They obviously both still love Assiah now; just have different ideas and such...
So - maybe they were very close brothers and loved to frolick around and mess with the humans a bit.
Then Christianity came, and eventually, the True Cross.
So, there's Mephy and Lucifer, being treated as gods - and then they become demonized and are hunted. May be fun at first, but I imagine it'd become tiresome after some time.
So, Mephy takes part in forming the True Cross Order to be an exorcist organization instead of a military one. He knows that they need to use different cultures in order to be affective. And, he knows they can use 'demons' to aid them - as it was done by the ''pagans'' before that anyways.
Hence, some demons are put in a bit more safety and Mephy can gladly enjoy his humans and not have to feel like killing them. Throughout the years Mephisto has added his touches to it....
Now, I can't really say when Mephisto officially joined the order, it's obviously been a fair while...but moving on to other things:
Now, we know Mephisto says he's been called ''Loki'', ''trickster'', and ''raven'' and he's been using Mephisto for around (or over) 200 years.
As I posted before:
The name can also be a combination of three Greek words: "me" as a negation, "phos" meaning light, and "philis" meaning loving, making it mean "not-light-loving", possibly parodying the Latin "Lucifer" or "light-bearer".
And then the Illuminati:
The movement was founded on May 1, 1776
Now, Lucifer's Illuminati may be newer but...
Going by that date saying that Lucifer started this organization then would possibly say that:
Mephisto and Lucifer came to a point in which they had a disagreement - the one of which we all know about - Assiah. After it became apparent they couldn't agree, they decided to go their separate ways and hope that one would give in.
I'd say this is a possible time when Mephisto joined the Order officially. He could now have the back-up of the exorcists to use, and still have fun in the meantime of course.
And Lucifer went off to create the Illuminati...it got ruined and all but he kept bringing it up out of the ground and such.
And now we are where we are at now.
And I'm going to bed...sorry if I missed points. Pretty sure I did...And I'll read those Archer I sweareth!