I thought of that, but I'm struggling to come up with a credible threat that
a) won't actually kill the Caliph or Ahm-La-i-m (because if the assassins can pull it off, hey, why should the PCs interfere?), but also
b) is dangerous enough that the Caliph and his guards can't handle it themselves.
If this guy is a Villain, I'd show his villainry off. Show the state of teh common people of Kequalak during the parade; utter poverty in the shadow of million gp homes, executions or sentences to the gladitorial arena for petty things like vandalism or insulting a nobleman, etc. Show why this guy needs to be killed.
Or, if he's not that bad of a guy, play up that he isn't that bad of a guy, maybe the PCs will be at least a little squimish over whacking him.
You could have the PCs interact with him - perhaps during some sort of sporting event. Polo, or whatever the sport of kings is there.
This might be too late as well, but I liked the idea of the arena. Maybe included in part of the festivities. The arrogant leader showing his prowess slaughtering Dragonborn (rulers/nobles from the region of the dragon he wishes to conquer). Maybe the assassin has provided the unarmed dragonborn with poisoned weapons, thus getting past the guards and remaining in the shadows.
Maybe the dragon attacks and the action takes place while the leader finds himself in a very tricky situation, trying to scramble out of the arena, surrounded by vengeful dragonborn, probably poisoned and many assassins waiting in the shadows. He orders his men to the airship to take up the fight with the Dragon there and then.
Perhaps the PC's were given a chance to win favour competing as gladiators previous to the leaders performance, so they are very close to the action when the Dragonborn produce the hidden weapons.
In the chaos anyone could be blamed for his death. And the airship and the dragon are still in it. Also with so much chaos the PC's will have lots of room to come up with an original plan of action.
I'm curious what you used to emulate the perform skill.The bard and the assassin jumped into the festive atmosphere, trying to show everyone how cool they were. The assassin impressed Ahm-La-i-m by integrating knifeplay into a belly dance routine, while the bard performed a retooled version of his earlier songs, now showcasing the assassin's beauty and heroism while battling dragons.
An on-the-fly ritual design?the PC wizard came up with a ritual to trap a portion of an elder god they'd crossed paths with earlier into the back of his mind

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.