You guys have provided some great ideas. My problem seems much smaller to me now than it did before. Thanks for that. Anyway, for those who are curious, here is the over-arching campaign that I had planned out. I'll try to condense it.
The Villain:
The main antagonist is a yet-to-be-seen vengeful god who started out as a member of a race (Armaranthine) that predates men/elves/dwarves. Originally the Armarnathine were the only creatures who could use arcana. The other races gained the trust of the Armaranthine until they were shared the secrets of magic. However, once enlightened, the mortal races turned their new found power against the Armaranthine and all but exterminated them. Morvek (by antagonist) was one of the few remaining Armaranthine. He convinced the gods that the mortal races, now enlightened with arcana, would eventually turn their strength against the gods themselves. Fearing for themselves, the gods made Morvek into a deity in his own right and blessed him with the power to keep the mortal races in check. Bent on revenge, Morvek began a reign of terror upon all mortals that showed no sign of abatement.
A party of five formed in answer to this threat (mixture of men, dwarves, and elves). After receiving blessings from a sympathetic deity, they eventually gathered the strength to push through Morvek's armies and challenge him directly. But even with all their power, they could not kill a god. Instead they banished him to a another realm using a spell that required their blood. After sacrificing themselves upon the portal alter they became known as the Quora Tal (Blood Guardians).
Skip ahead to present day and Morvek is seeking to free himself from his prison. To do so he must collect the direct descendants (first born of the first born... and so on) of the Quora Tal and sacrifice them upon the alter which originally banished him. Of course he cannot do this himself. So instead he sends minions to accomplish this task for him. So far my players have encountered one such fiend and defeated her. It was from this adventure that they discovered the existence of The Reckoner (Morvek), the Quora Tal, and they vaguely understand that The Reckoner seeks to free himself from his prison and that he must have the blood of the Quora Tal to do so. They know practically nothing of who/what Morvek is or what his plans are aside form freeing himself. My intention was to have my new player (he is a bard) reveal to the group Morvek's true name, and also the names and classes of the original Quora Tal. Oh, and he is also aware of the identity of one direct descendant (see below for more on this).
Original Future Plans
With more questions than answers, my players were on their way to the only metropolis in the continent they are playing on (it is also the arcana capital of that continent). They were hoping to gain an audience with some of the higher level mages in the Arcane Order and see what they can learn about Morvek and the Quora Tal.
Along the way (it's a two month trip to get there), they ended up rescuing a male elf in the forest who got in over his head during a hunting trip. They learn that the elf they rescued is actually the crowned prince of the country they are headed to. It was at this point that my bard was to reveal to the party the fairly common knowledge that this prince (and his father) are the only known direct descendants of the Quora Tal. Their royal line descended from the elf sorcerer who originally helped to lock up Morvek.
This is where the trouble began. My new player then went on to tell my group not to trust this new NPC because it is "rumored" that he or possibly his father is an agent of Morvek himself.... That wasn't supposed to be revealed for a VERY long time. After the session was over I asked one of my particularly smart players to tell me what he thought of all the information that was inappropriately revealed and he guessed my plans to a "T".
My player guessed that the prince is likely unaware of his father's connection to Morvek and his father (the king) has probably made some deal with Morvek to sacrifice his son when the time comes in order to gain some sort of favor/power.
Ding* Ding* Ding* We have a winner!
The focus of my campaign was meant to be for my players to track down all the remaining descendants of the Quora Tal and decide how to prevent Morvek from accomplishing his goals. Gradually I would have revealed the details of Morvek's back story as well as what his likely plans/motivations are. I was hoping to make Morvek a fairly sympathetic villain and let that play out how it may. My problem now is that my player who already knows everything will not appreciate the reward of information over time. On the bright side, he didn't really reveal the back story to my players, just my plans for future plot twists.
I had wanted my players to befriend the prince and let the prince be a useful resource to them throughout the campaign when necessary. When the king eventually betrays the group by betraying his own son, I was hoping their friendship with the prince would make the event feel all the more personal. I wrote the prince to be a good man that any king would be proud of as a successor, but for some reason never seems to gain his father's approval. The dismissal would be obvious to most everyone though few understand the real reason why the prince cannot please his father; it is that his father sold his son out as a young child and therefore has never had any intentions of investing in him as an heir.
What now
After reading all your advise, I suppose my blunder isn't likely as large as it felt at the time. But as you guys can see, I'm likely going to need to work out some new intrigue as well as a big "Twist" moment as the campaign winds up for the ultimate climax. I liked this campaign idea. I'm not sure how original it is, but it was the best I could think up with my limited imaginations. My favorite part was that I could probably make it last for years due to the difficulty I imagine my players will have in just trying to find the heirs of the Quora Tal. And then of course they have to decide how to solve the ultimate problem which is thwarting Morvek for good.
I'd appreciate any input you guys have for major or minor adjustments to this story. I like to tell stories, but at this point I'd prefer to focus on world building and planning colorful sessions. I'm tired of toiling over working out the big plot points for this campaign.
If you read all this, you're a rockstar