I'm no authority on the subject, but I think you'd be hard pressed to find an actual resurrection ritual beyond fiction. On the plus side, you don't have to base your own fiction on somebody else's ideas. Your own ritual can be whatever you want it to be. There may be some archetypes involved, such as those mentioned by jefferytitan, but nobody will hold it against you if you stray from previous formulas.

Rituals are only a detail of the plot, and in a game they should be designed according to practical criteria because they are completely arbitrary:

• What is suitable for depiction? For example, chanting in made-up languages, chalk diagrams and candles might be rather family-friendly, violent human sacrifices in the middle of a disgusting sex ritual probably not. What pretty graphics and special effects are doable? How can they be adapted to be part of a magical ritual?
• What is suitable for player interaction? Assuming the cult has to be disrupted, means could include sneaking an "impure" person inside a sacred perimeter, damaging an artifact, distracting or attacking cultists at a critical moment, delaying cultist travel, suppressing information, destroying or defiling a place of power, etc. The genre of your game determines the genre of the rituals; if the action is fun, any excuse about how magic works is acceptable..

On the other hand you should reflect on the higher metaphysical level on which your plot is supposed to make sense:

• What are gods?
• Are there even higher powers, e.g. generic "good" and "evil"?
• What does it mean for a god to die, and revive?
• What sort of magic is possible? (Reviving dead gods is probably one of the smallest niches of magic, probably something that never happened before; there has to be some larger body of tradition and knowledge.)
• Where does magical power come from?
• How can mere humans interact with gods?

For example, requiring the death of another (enemy?) god to resurrect the old dead one might range from appropriate because of cosmic balance and limitation mechanisms, to a reasonable plot gimmick to make resurrection difficult, to preposterous because the death of the god was a very special occurrence that has nothing to do with other gods.

Well you see, it's actually quite simple. You just go and...http://i0.kym-cdn.com/photos/images/masonry/000/770/990/04a.png

While I do agree with most of the points above, I don't think a horror game meant to be psychologically scary has any real need to make sense. Reason is one of the factors that helps us rationalise things, not to mention that humans are afraid of the unknown (extremely so). One of the great things about focusing on cult's is that, generally, what they do does not make any sense. Beyond the whole 'sacrifice lesser beings to summon/appease greater beings', most sacrificial cults just do not make sense at all.

I noticed how you said you wanted to be 'somewhat' authentic. To be honest, occultism isn't widely known, or even thought of, beyond the sacrificing. People only see a cow getting massacred and say 'wow, that's barbaric' which is ironic but that's going off-topic. What I'm getting at is you should only really be looking at going authentic with something if you believe that your audience has knowledge of the topic. Unless you personally feel the need to.

In the case of occultism, I'd say research 'sacrifices' and make that authentic. Anything else you can either draw some inspiration from real world occultism or make it up completely. Note that when I say sacrifices I'm not including resurrection, I believe that resurrection is something that varies from cult to cult, so there's no real need for it to be authentic. However, you may disagree and want to have a lot of the cult workings authentic, so I'm providing some material to look at:

http://articles.chicagotribune.com/1988-08-21/news/8801240414_1_santeria-animal-sacrifices-animal-regulation-board

http://www.davidicke.com/forum/showthread.php?t=109379

I found various other ones in my research, but all of them were either fictional creations or people disgusted at cults (not really explaining them, rather degrading them). Interestingly, or rather logically, people seem to avoid the subject all-together. it's very hard to find a reputable source of actual cult in-goings, and no I don't believe its because cults are very secretive (although it could very well be).

What I could suggest is going onto the cult wikipedia page and looking for tangents. It'd be your best bet of finding cults, but it'll also be very time consuming.

Let me wrap up by repeating what I said previously, reason is one of the things that makes a game less scary, so don't worry about how much sense it makes. Just focus on what you want it to be like.

Side note: I think it'd be cool if the game starts where you wake up and you're chained to a stone table with a ritual being performed around you. You see dead body's strewn around the place as you look around, before you look down on the table. The tables completely covered in blood. Then you frantically look up to see a knife plunge into you and you black out again. Next time you wake up you're among the dead bodies, and the ritual is still being performed with more people. You were fortunate enough for them to have missed your heart, but to be physically dead for a few seconds. This is your chance to escape.

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I had a chat with my mother who did some research on occultism. She said to read between the lines. Think of what's posted online as a net. A lot of it is their in order to find new recruits. They're seeking people crazy enough to join their cult, so they post things similar to what their cult actually practices in order to find people interested in it.

But besides that, their is not much you can find online. These cults truly believe in what they're doing and are very close-mouthed about it, you're either with them or dead. Your best bet would be to join a cult itself, but I highly recommend you do not. Not only would it be extremely dangerous, if you couldn't stomach it you'd be found out immediately.

Side Note: She said that as she has not had first-hand experience in a cult she cannot guarantee this, its possible that some cults let their members leave on the condition that they do not reveal any information. But in that case, you wouldn't find anything out form them anyways.

She suggested that your best bet would be to go to a library and look up certain key phrases like 'summoning' or 'sacrifice. A lot of the information of cults are old historical documents detailing certain practices that happened in their time. If you fail to find much in your local library, it might be worth travelling to a larger library.

Also, this is a forum focused specifically on occultism that I found: http://www.occultforum.org/forum/index.php?sid=eb9514ea4248849b599cf6993b146041

It'd be a gold mine for real cults to grab fanatics, so I'd say there is a lot of material their that originates from real cults.