Ooh, a short one. Only 32 pages. It's a complete adventure, but it feels rushed. A lot of stuff happens in the first encounter and the PCs have a narrow window to get on board with it. The result is something that, if it works, it'll work, but if it doesn't, the players are going to bounce right off it, and the only way to know which scenario is going to happen is just to jump right in and try it.
Backstory time - there's this monadic deva (an angel, basically) who was assigned the task of infiltrating the Abyss and spying on one of the demons there. But he has a problem, his angelic light is so obvious that he'll be spotted instantly. His solution? Remove the part of himself that gives him his angelic powers (the titular "deva spark") and stash it in some worthy soul until he finishes his mission. So he pops over to the Prime Material plane and finds some rando who looks like he's doing a good deed - saving a young girl from a burning building. But it turns out that the guy was really the one who started the fire in the first place, and though he lacks the vocabulary to describe the sudden burst of power he got from the deva spark, he does know that he's going to use it to do a whole bunch more arson. Then he promptly dies, shot by the homeowner whose house he burned down. His soul, along with the attached deva spark, goes straight to the Abyss.
This is where the PCs come in. Sort of. There's still more, but the start of the adventure overlaps the end of the backstory. The PCs are in a Sigil bar (you can tell because there is an aasimon and a tanar'ri having a drink together), when their sexist stereotype of a waitress ("generally being a nuisance - in a cute and ditzy kind of way") gives them the wrong change. One of their coins turns out to be a gate key.
A slight digression - the DM's notes for this exchange are wild. "Among the coins is an electrum ingot with strange markings on one face - a crude infinity symbol pierced by an arrow pointing down. The other face is blank. . . The nature of the coin shouldn't even be hinted at by the DM. Don't point out its strange appearance. Don't single it out as an item of interest. Just let the PCs pocket their change and be on their way."
With this strange coin in their possession, the PCs carelessly walk through a portal, finding themselves on the Abyss!
In another tavern. And who else should be there but the disguised deva. It is at this point that the soul of the dead arsonist barges in. He's being chased by a demon! The deva recognizes his spark and tries to fight off the demon, but is mortally wounded by creature's poison (because the bulk of his power is inside this random dude). The demon eats the damned soul, deva spark and all, and the backstory officially ends with it fleeing into the darkness.
Now, if you're like me, all of the above inspired three wrong guesses about what this adventure was going to be about. It is not going to be about stopping an angelically-powered super arsonist. It is not going to be about scouring the Abyss for this one specific soul. And most surprisingly of all, it is not going to be about tracking down this demon and slaying it to recover the spark.
It's actually all an elaborate setup for the convoluted moral dilemma in the final encounter. The deva can be healed if it is reunited with its spark, but he insists you escort him back to Elysium so he can report his espionage info to his boss. Luckily for him, the demon is gradually being corrupted by the spark, becoming a creature of good (this manifests by the demon killing random people and then feeling guilty about it), and it heads to Elysium on its own initiative to attempt to understand its bizarre transformation.
Yada, yada, yada, after you and the deva and the demon all team up, the sages of Elysium tell you that you must seek the three plot coupons and take them to the heart of the allegorical forest, wherein you will be presented with the power to determine the fate of the deva spark - leave it with the demon, who is now "good" and let the deva die; give it back to the deva, "killing the good creature the demon would become;" splitting the spark in half, so that each of the recipients becomes a diminished version of its full potential; or merging the two celestials into an entirely new being (i.e. the decision the adventure has been unsubtly telling you is correct).
It's not as railroady as it appears, because there are a lot of failure points along the way and the adventure is generally willing to let those failures play out (although some of the choices are pretty inexplicable - like what happens if you attack the angels in order to protect the demon, instead of just explaining the situation). I guess if I were to characterize it, I'd say it's a high-wire act of persuading the PCs to care just the right amount. If, at any point in the adventure, they get a strong idea about the importance of any particular goal, that is going to wildly change how they approach the adventure's obstacles and the story will quickly veer off the map. Similarly, they are not getting paid for this and none of the events or characters have anything to do with them specifically, so it's both tempting and easy for them to walk away at literally any point in the adventure.
So, I don't know. I guess I have to give Deva Spark points for trying something original, even if its dilemma is extremely weird ("the demon grew an organ that lets it feel guilt, so now it deserves to live"). I don't think it will actually work in practice, but it couldn't hurt to give it a try.
Ukss Contribution: Honestly, I think the first implied adventure, with the angelically-supercharged arsonist, would probably be a better adventure. And even though the opening fiction stated that he saved the girl more or less by accident, I think a guy who starts fires specifically so he can get the godlike rush of "saving lives" would be a damned interesting villain.
So that's what I'm going with. A god-ridden arsonist who starts fires to set himself up as a hero.