One of the best triggers in D&D games for me has been names. These usually turn into NPC names or location names but sometimes they turn out to be something more.
I ran a campaign called Kilranthia at +ORC Edinburgh which had elements from a previous campaign that I'd ran; specifically I took the names of PCs from an old campaign and built them into the pantheon of Kilranthia. Now that was the easy bit and wasn't actually the trigger, it just helped me to refine the trigger into the fully formed one that Kilranthia needed.
The trigger became known as Dimura's Prophecy and this is what it said -
The life of the world began when the light of the sky first shone upon the child of the land.
From the heart of the forest came the hand of the destroyer and he stole the eye of the mountain.
The mouth of darkness tried to remove the jewel of the ocean and block out the light of the sky.
From the child of the land will come those to face the hand of the destroyer and free the eye of the mountain.
In the heart of the forest hope remains.
In the eye of the mountain hope depends.
In the child of the land hope relies.
Without the light of the sky hope dies…
In the mouth of darkness despair waits.
In the hand of the destroyer despair triumphs.
In the jewel of the ocean despair fears.
Without the life of the world despair nears.
Let's take 2 of the "names" that are referenced and see how I used them in the game.
"Eye Of The Mountain" - Represented in the game by an eagle named Lóthdor.
Lóthdor was an avatar of one of the ancient and forgotten gods. This god was largely nature based and had created Lóthdor to be his Eye so that he could watch over the world and help guide those in it. Essentially all that happened here was that one of the PCs had a companion who was an eagle named Lóthdor who also just so happened to be an avatar of a once dead god.
Lóthdor was both a distraction and a focus for the characters at different points in the story and became a key device in reminding the players that the prophecy was tangible and could be used to influence them.
"Hand Of The Destroyer" - Represented in the game by a big time Wizard named Weston who worked both angles whilst at the same time looking to grab all the power for himself.
Weston was nobodies fool and someone who worked all the angles to get what he needed. He was both the saviour of the PCs and their enemy. Ultimately he was manipulating the world so that when that moment of The Big Bad winning was upon them he was ready. Essentially he was there to emphasise the doubt the PCs had in what they needed to do and no matter what The Big Bad did, Weston was always going to win. If The Big Bad won then Weston was the reason for that success. If The Big Bad lost then Weston again was the reason for that failure. This really came down to who the PCs trusted which in the end it all came down to a coin toss...
Some of the above will read like railroading and at times I'm sure that did happen however I'd suggest that those instances were more about demonstrating that forces beyond the understanding of the PCs were involved in the plot. The role of the PCs was to change the balance of any outcome, to be that thorn in the side of The Big Bad and the prophecy gave them the means and opportunity to do that.