Some back story.
A war is brewing between two kingdoms, and the players are aware that other external forces are gathering strength and may be plotting to take advantage of the conflict. As a result, the players are now working to figure out how to prevent the war, gathering allies, and so on. A lot of stuff has gone on blablabla. They were tasked with protecting the young prince of one kingdom as he was to be betrothed to a princess from the other kingdom. It would prevent the war.
So they accompany the prince, whom the party and some royal guards protect. Unknown to them, the driver/guide is a traitor and leads them down the wrong path to an ambush. The party failed all their dice rolls to notice it.
The ambush occurs, the driver cuts the harness for the horses, jumps onto one of them, and tries to ride off.
The players noticed and moved out to take defensive positions, and they managed to stop the driver before he got away. They failed to spot the enemy assassins, but do they know something is wrong?
So the players do everything right. Some go to secure the Prince, some go to capture the driver, and some are moving to intercept potential enemies.
The prince is in hiding, but looks out to see what is going on, hiding in cover, and barely peeks out. The main assassin leader fires an arrow at the prince. Now, I designed this encounter to be where the prince has enough HP to take at least one hit with the maximum amount of damage from the assassin. The arrows have some poison on them, so if he is hit, it would add some extra "oh shit" drama to an encounter. Or the arrow would hit the side of the carriage and.
However, at this point, the dice decided to fuck with the players. I rolled a natural 20, and not only that. I rolled almost max on every damage die, so the prince was dead in 1 hit. BOM, the party is in shock. Their last 2-3 months of sessions' goal is now dead.
Well, shit, combat starts in earnest. Assassins and their support attack. There is a massive fight.
The players cut down the Assasins, taking some damage, but unknown to them, a secondary goal for the Assasins is to steal a royal artifact, a goblet that was part of the dowry to the other kingdom.
During the last 3-4 rounds, a low-level assassin sneaked his way to the carriage and was not spotted once by passive perception. He managed to sneak onto the carriage, break open the lock box, and get the goblet.
That is when he is spotted. So the players are all surprised to see this man on the Carriage, and what next is pure elfbowl nonsense.
The players do everything to stop this guy as he makes a break for it, fleeing away from the party towards the leader.
The players do everything right; they are putting the hurt on him, using spells, and everything else. Again, the dice were not to their liking, and the assassin managed to stay standing when needed, with only 2 HP left. So, in one last Hail Mary, he throws the goblet to the main assassin leader, who picks it up and runs away from the fight.
Now there is still one loose end. The Driver is still alive; he was low on HP from an explosion, but alive. If they can capture him, they may be able to gather some information about who is behind this.
The Driver is too far away for the assassins to cut down; the assassin leader was the only one with a ranged weapon left, and he was running away while carrying the goblet.
So the players are about to knock the driver out and capture him. However, there is one more enemy left, not an assassin, but a soldier/defender type. He is nowhere close enough to run up to the driver and kill him; he had lost his sword earlier to a command spell. However, he had a shield, so he used an action to remove it from his arm, another action to move, and then a third action to throw the shield at the driver.
Very, VERY low chance to hit, with the penalties for an improvised weapon, the very long range I said that only a natural 20 would hit, and what are the chances I would get that again in this crucial moment? Well, I did, and that shield sailed through the air, hit the driver in the head, and he was dead.
I was laughing hysterically over the situation.
In the end, the players had lost the prince, all the royal guards, and the carriage was on fire (assassins had firebombs). The main assassin had escaped with something they did not know about, and the last lead was that the driver was dead.
The players did play well, but the dice decided to set things in motion in this way, and it was great. I am looking forward to the next session as they try to track the assassin down to figure out what's going on, or better yet, can they salvage anything?