Almost any epic level threat will do, but he/she/it should arise as a direct/indirect result of player character meddling (thereby making them responsible to destroy it). Preferable if PCs have a choice in what they awaken, but don't realize it. (think primordials, demon lords, gods, etc...) (it's only railroading in the sense that they'll probably have to fight this baddie towards the end of the campaign).
Not quite awakened, but certainly it's indirectly their fault: An old wizard hires the party to find an obscure item/book in a dangerously guarded treasure horde so he can destroy it to stop anyone else using it for evil (entirely true, with that wording!); a few clues in the horde point to the fact that said item is rather dangerous; six months later a lich starts it's reign of terror, the wizard having finally got the component he needed to complete his lich spell.
Fantasy literature and fantasy themed tv shows and movies are filled with this trope, so there are plenty of examples to draw from.
Killing one evil releases another. This is all over the place, but one fairly recent example was when Sam and Dean killed Lilith and thus broke the final seal to begin armageddon in Supernatural, I think it was season 4.
Disturbing a tomb The "graverobbing unleashes ancient curse" is a tremendously common trope, with a recent example in "The Mummy". Depending on your campaign, they might be graverobbing purely to get money/loot and might even know it is cursed (but drammatically underestimate the power level). That works well if you want to show the consequences of greed. But they might be doing it for some greater good if the tomb contains...say the cure to a magical illness that the princess has been afflicted with.
- Making a bargain. Bargains with evil entities is a (literally) ancient trope. Marlowe's Dr. Faustus provides an example of someone who makes a deal with a devil for selfish purposes and is destroyed by it. But in more convoluted settings, the characters might think they are doing it for the greater good. This happens more than once in Supernatural. In The Adventures of Merlin towards the end of season 2, Merlin strikes a deal with a bound and caged dragon to free the dragon in exchange for the Dragon's help in savin Camelot. But Later the Dragon becomes enraged and seeks to destry Camelot himself forcing Merlin and Arthur to fight against the Dragon.
- Angering someone that unleashes the evil. This is a little more indirect, but if the player characters enrage a powerful wizard/cleric/etc. That wizard/cleric could deliberately summon an ancient evil to take revenge on the characters. Of course in that case the characters might not feel directly responsible since it was the evil wizard/cleric/etc. that did the summoning on purpose, but it was still done in response to something the characters did. This is common throughout The Adventures of Merlin where several threats have been summoned by enemies of Uther seeking revene for things Uther did in the past.
- Enabling the summoner Similar to number four, the characters might do something that lets an evil wizard deliberately release a greater evil. They might bring him a hard to find and necessary component for instance. They could do this unwittingly, either to finish a job and get paid or because they believe that what the wizard will do is for the greater good. In Aladadin Jafar tried and eventually succeeded for a while in getting Aladin to bring him the lamp to unleash the genie.
One AD&D campaign I was party to included a small menagerie of magical sentient swords, each capable of defeating a different archdevil that is imprisoned by the power of another. Because of the renown for the swords with a certain villainous cult, archdevil #1 was released and we had to put it down. Because one was released, the other started to awaken so the party opted to go around and try to wake them up from their prisons and put them down before they had their morning coffee, one by one as we had (what we thought were) the right swords.
Another, less convoluted, method is that the players are on a treasure hunt and things go a little Mayan Temple / Egyptian Tomb and the way back becomes blocked and the only releasing mechanism is the complete chickenswitch that summons the Torrasque du jour to the Prime Material and puts a bee in its metaphorical bonnet.
Do something innocuous - eat the wrong thing, say the wrong thing, unknowingly open pandora's box, trip over the wrong wire, press a red button, mispronounce a magic word, etc...
Raise a demon with every intent to kill it. Try to strike a bargain. Make a wish with side effects.
Possessed by a demon, sent in by a doom worshipper under pretense of something else, thinking raising a force for good
Unalterable (ie, happened in the distant past but just now discovered) series of events leads to evil being unleashed. Ie, character was born, etc...
Demon possession of a force for good, cause an angel to fall, etc...
Force of nature
Accidentally break gravity, start global warming/an ice age, etc...
Inspired by the "Mistborn" series by Brandon Sanderson the adventures could go on a quest to rid the world of a powerful tyrant by gathering artifacts that will help them defeat the tyrant.
The catch is...spolier alert
the quest's instructions had been corrupted so that the assembled artifact actually unleashes a more powerful tyrant, in the case of the book series, "Ruin" whose only interest is in destroying all life. Then, the adventures have to work to truly understand the quest's instructions to defeat the life-destroying tyrant.
so here we are in the ancient chamber, rune-encrusted walls and columns surround you and there's a large round pattern laid out in the centre with a red centre and rune surrounding it that read "idle thumbs".
I go over and look at the bit in the centre.
You see an inscription "don't press".
There are plenty of other similar ideas - from Indiana Jones picking up the gold idol, and unleashing the boulder, to John Hurt "having a quick peek" in the funny egg-shaped object, to Ash saying "Klaatu... verata... n... Necktie. Nectar. Nickel. Noodle". (I quite like this last one, tell the players the magic words, without letting them write it down, and then ask them to remember them much later on. If they get it right, the wizard telling them can be 'absentminded' and told them the wrong phrase in the first place!)
Tempt them with greed or lay a hubris trap. For example, a tomb that, at least culturally, should be somewhat taboo for them to touch, yet they are tempted to open it up for the riches that it contains. Too bad the purpose of the tomb of St Fred was placed on that spot to seal off the evil contained below it.
A rather simple answer, and one that is not foolproof, but just allow the players to be somewhere they shouldn't be.
Maybe an area they are exploring for an unrelated reason has a wall that crumbled due to natural (or supernatural) phenomena, behind this wall is an area that has been untouched for centuries with all sorts of nick-knacks, unpressed buttons, macguffins, strange treasures, whatever you think would be most irresistible to your players without you having to tell them to go in.
Once you've done this, with no direct hook other than opportunity, what they do to awaken the evil is entirely up to what they did of their own volition. Not only does this work as characters but also in what they did unbidden as players. In this situation you can have several potential triggers set up and you can choose the one that makes the most dramatic sense depending on what goes on (If you've watched Cabin in the Woods you'll have seen an example).
Even if they do absolutely nothing and refuse to even go in, you can still have the evil use the fact they are present and nearby living beings as a guide for how to get out. It's less their fault and more "Wrong Place Wrong Time" but it gives you a nice fallback if your players prove to be beyond temptation.
Players are very willing to break into most anything without regard for what might be sealed up in there. I see no need for anything more convoluted. The warders of course put dire warnings but they don't say what it is because they don't want to let anyone who is seeking to free it know where it is.