In my plot Orcus has his minions try to collect the pieces of a gate spell to summon him from the Abyss. The climax comes after defeating the leader of Orcus's cult, the cult leader used the spell before the players got there but wanted to kill the players herself. She is pretty much the final boss and once defeated she dies the spell goes off and a portal opens. 

Orcus can't leave the Abyss himself so he just asserts his control over another player whose story was all about being possessed by a demon.. 

For this ending to work I need to railroad not just the spell but also everything leading up to it. The cult needs a gem, a location, and a copy of the Gate spell. I thought the players intervene between with the cults mission only for the cult some how to winning the object. The players would save people from the cult when they try to kill people but they will still et their prize. 

I want to know ways I can make the railroading seem fair. I don't want the players to feel powerless to the DM and his reckoning. How do I make this story feel fair?

I would advise you not to railroad your players. You will end up with the players feeling powerless. In particular, the whole "no matter what you do Orcus is going to be summoned" thing -- and trust me, I have done stuff like this in my day as a younger GM back in high school -- is not going to be satisfying to them. You will not be able to conceal the fact that no matter what they did Orcus was going to be summoned. And it's not just this -- it sounds like everything along the way is going to happen no matter what the players do. This isn't a D&D adventure (or campaign) -- it's a scripted play. And yes, I have run adventures like this before... and yes, it can go OK, if, the players know and agree that it is going to be a strong story. But you are not going to be able to conceal what you are doing from the players, and there is no way you are going to make the railroading "seem fair." It will seem monumentally unfair to them, and they may well become frustrated (again, unless you are in a case in which the players have agreed in some sense that the story has a fixed trajectory and they are along for the ride).

I recommend instead that you design a generalized plot for the bad guys -- that is, you have outlined what the bad guys want to do. And then you set up a timeline of when they will attempt to do it. At points along the way, depending on what the players decide, they may be able to intervene or prevent these things. That's fine -- the bad guys can have back-up plans. The cult needs a gem and the players happen to get it? OK, that's fine... then the players find themselves pursued and harassed, perhaps being attacked by increasingly strong servitors of Orcus.  Or maybe, the cult just needs the gem to be at the location and it's not so bad if the players bring it there. Maybe the cult can find a way to trick or coerce the characters to go to that location. What if a friendly NPC the players like very much got kidnapped, and a ransom note said, "Bring the gem to the top of Cult Mountain by June 1st, or the halfling gets it."

This will give the players choices, decisions, and options. They could choose not to go, deciding that although the like the NPC, protecting the world from Orcus is more important. Then the villains will have to (a) kill the halfling, and (b) find another way to get at the gem. Perhaps the players will go, but try to do something else, like ambush the kidnappers. Either way, what matters is the adventure the players are having.

I suggest that you not fall too much in love with a climax scene you have written. It may well not work out that way. I'm planning a campaign set in Ancient Rome, and if I ever actually get players and do it, there is sort of a "world plot" going on that the players might, perhaps, decide to involve themselves in. (I will give them lots of entry points to do so, but it will be up to them to decide to engage or not.) If they do not get involved, over time, a powerful enemy of Rome is going to develop the ability to attack it, and sack the city and possibly conquer the Empire, causing its downfall. If the players choose to engage, well, they may discover that Rome, although it is their home, is not so wonderful after all, and they might side with the enemy, choosing to assist in bringing about that downfall. Or, they may decide to defend their home and try to stop the bad guy. I don't know which one they will do. It will be their choice.

I suspect, that having come from Rome or its allies, and probably being good (again, with a party that doesn't even exist yet and might never exist, since I am just sketching the ideas now and not starting a campaign), with the enemy of Rome being, perhaps, originally justified in his hatred of them but now an evil servant of the Dark Side along the lines of Darth Vader, the party will probably choose to engage and likely try to stop the enemy. Assuming they do that, I have a very cool climactic scene in mind, including possible soundtrack music I could use at certain points. But this would only happen if (a) the players choose to engage the "world plot" rather than just go on random adventures, (b) the players decide to oppose, rather than assist, Rome's enemy, (c) the players find all the clues they need to achieve their goal, and (d) the players defeat said enemy in some way. That's a lot of ifs... and without all of them, my really cool climax scene won't happen. As a DM, I need to be okay with that.

After all, I could always turn the campaign into a novel and write it the way I am imagining (and play the music I am picturing for the "climax scene" while I write it) to get things to 'come out the way I want.' But I don't think it's appropriate to try and force the hands of the players, or put them into situations where no matter what they do, the DM's will occurs and their choices or actions don't matter.

Agreed. I’m writing a very similar scene into my campaign right now, only the motivation for the scene is one of the PCs back stories so that PC is going to drive the group into the scene, and I’m only writing the opening of the scene and the goals of the NPCs. The PCs are going to write the rest of the scene from there. In the spirit of “Let the dice decide”, if the PCs don’t stop the summoning it will be completed and the god that is being summoned will take over the body of the wraith Necromancer who is performing the ceremony. If the PCs do stop the summoning they’ll destroy the wraith Necromancer and be left with the ritual book that contains the ceremony and a decision, do they destroy it or do they do something else with it?

In my experience the DM sets the stage, but the PCs should write the story. As a DM I set up the scenario and the motivations of the NPCs. Once the PCs start interacting I determine the reactions of the NPCs, but ultimately the players decide the outcome.

That’s why I never say “in my campaign” but instead always refer to “the campaign I’m DMing.” It’s their campaign, I’m more of the narrator.

How close are you to this finale?  Are you intending this for your next session or are you just mapping things out?  

As others have mentioned above, the scenario you outlined reads much more like a script where the players don't have any ability to make a meaningful difference.  You know your table, so they may like this,  and if that's the case, then feel welcome to ignore the advice below (well, you can ignore it anyway if you want)  :)

You have a setup.    Villain:  Cult leader    Objectives:  Get a specific gem;  find a location for the summoning;  have a copy of the gate spell.

If the cult leader doesn't have the gate spell, this is all for naught, so in an upcoming session, when next the characters encounter cultists,  provide a way for the characters to learn that there (a) is a cult leader (b) they are working on summoning orcus.   This can also add a time constraint since the characters may not know how soon.

As the party then plans to confront this, through checks, investigation, RP,  or other means have them learn that the leader needs a focus for the spell, perhaps a gem and an appropriately powerful location to cast the spell.  The cult leader may be able to do it without these things, but those items greatly increase their chance of success, so they are sending minions to scout/find the above.  The characters can then make a plan and determine how they want to solve this problem.   

You can have multiple sessions with either dungeons, rp encounters, or information gathering to get this information to the characters (based on character choices).  Perhaps even add an item or a visual cue like a countdown timer so that the characters feel a sense of urgency in stopping this, otherwise they may say, "Oh great, a demon lord is being summoned,  let's go to Waterdeep and tell them so that a higher level group of folks can deal with this".  The visual cue could be a number of spots that appear on the moon, or maybe dark stars surrounding the moon.

TLDR:  You have a villain and you know their motivations and what their objectives are.   Provide ways for the characters to learn of some of those objectives so the characters can choose how to approach the problem.  

Hope the above helps!

I read once a great piece of advice which is to setup a storyline / timeline of what is happening, why, and what will happen as if the party didn’t exist. Then throw them into the mix. You then modify what will or is happening based on what the party does.

By understanding events that are in motion and the motivations of your NPCs, it’s pretty easy to adapt things based on the actions of your players. You can give forethought to a surprising amount of this. I have a little “outcomes” section when I plan encounters where I give some thoughts to how the story might be affected based on possible outcomes (BBEG is defeated, runs away, defeats the party, surrenders, befriended, whatever seems likely). The party will occasionally end up somewhere else, but you can just wing it in those cases. 

Okay I came up with another idea, note that I am still in the writing phase so a lot could happen. Demons get stronger the more they kill, and I also thought that maybe the cult already knows that Orcus has taken a host in the world but just not whom it is. 

So my idea is that the plan is to use some sort of epic spell wipe out a whole city of people, the following death and destruction caused by the Relentless Sons (Name of the cult) fuels Orcus and makes him stronger while also furthering his desire to destroy all life. The cult means to do this through a old spell book which was lost to time or something. The only way to destroy the spell book is to burn it in holly flames. What do you think?

You could pull liberally from Supernatural for this. Orcus and other demons could hop between bodies which they wear out the longer they are in. This would let you out a very powerful demon in a weak body to have earlier encounters with them at lower levels.

Maybe throw some ambiguity in and have the city destroyed in a botched attempt to stop Orcus. The party might start out thinking the group responsible are “bad” (and maybe they are), but find out over time that they are trying to stop Orcus.