A special thank you to gigabein for their assistance.
For DM eyes only. Players should not read on.
A man is fleeing some templars in any city-state and bumps into the party, literally. No words are traded and the man flees. The templars catch up with him and kill him.
They search the man's body and ask "Where is it?" The templars will go search to see if he dropped whatever they are looking for. If the players get out of the area, they will eventually discover that the man slipped a rolled up piece of parchment into one of the characters bags.
The parchment is a map. The map has some characters on it and shows a path out of the city, toward the closest mountain range. If the characters can read whatever language you decide, the map reads: "We will gather here, in the <Enter Mountain Range>, no other location is sufficient."
Hopefully the group's interest will be peaked.
The letter is from a group of magma clerics, who are gathering in the mountain range to magically awaken a series of dormant volcanoes. They don't care that a slave village is nearby and would be buried in liquid hot magma.
The magma clerics have pooled their resources and hired a group of thugs and mercenaries to defend their ritual. The clerics will know that one of their number was killed by an outsider and that his letter his missing. If the player characters start poking around, the clercis will find out and try to have them killed.
The climax of the adventure should be an epic fight between a number of magma clerics equal to the number of player characters and twice as many thugs. The magma clerics levels should be an average equal to the highest level player character.
If the players succeed, no one will really know and the players fame will not proceed them. They will, however, gain a good number of magma worshiping enemies.