The king managed to acquire a huge amount of gunpowder he could possibly destroy Paris with. You must find the cargo and decide of this dangerous weapon's fate. This page of our Steelrising guide contains a walkthrough of the Gunpowder trail quest.
You'll receive this mission at the Cordeliers Convent, after completing the To Les Invalides quest. The questgiver is Lavoisier, who'll give you the quest if you ask him about what to do next.
He'll direct you to a large gunpowder depot and ask you to sink it. La Fayette will butt in in the middle of the conversation. He'll ask you to give the gunpowder to him.
Head to Les Invalides. From the square where you'll arrive via the Horseless Carriage, go through the gate to the left.
Immediately behind it, to the left, is a depot. Examine the corpse (bottom-right corner of the room, right next to the entrance) and the barrel marked with an X. Though the game at first suggests you find four clues, these are enough to continue. Your next destination is the Bastille.
You'll have an opportunity to complete this objective during The Prisoner in the Bastille main quest. After entering the prison you must go through the cells. In the room behind them you can unlock the gate to the right.
In the corridor turn to the first room on the left. You'll find a letter and a key on the desk. You can use the key to access the sewers.
Get there by going straight down the corridor, past the Vestal. In the room behind her there's a breach in the wall. Jump through it. Try not to fall to your death (you can dodge or use The Selenite Momentum right before landing).
The underground entrance is right opposite of where you landed. It's hidden behind a breakable wall in the riverbank.
In front of you there's an unlockable gate - a shortcut leading to the section of the sewers you were in before interacting with the Vestal.
To reach your destination, you must turn left in front of the gate - go down. Get through the room. On the other side there's a destructible wall. Follow the path. You'll run into a Lumberer (more about them on the Labourers page). Keep going down the corridor. Go down the stairs.
The first room in the lower part of the sewers holds a Harvester (more about them on the Serpents page. Enter the short tunnel behind this room and pull yourself up through the hole in the ceiling. Open the gate behind you to unlock a new shortcut. The main path leads through a wide tunnel and ends with a staircase leading down. You'll encounter a Miner and a Courier on the way (more about it on the Arachnids page). Go downstairs and open the gate with the key you got from the Bastille.
In the center of the room there's an elevated platform and the mechanism you'll use to decide what to do with the gunpowder. Approaching it triggers two possible interactions: leaving the mechanism alone or using it. Leaving it alone fulfills la Fayette's request. Using it destroys the gunpowder, as Lavoisier asked you to. Regardless of your choice, you must return to the Convent and report back to the questgiver. You can also open a chest here for a Carriage Token. If you're currently playing through The Prisoner in the Bastille quest, you'll have to finish it before returning.