What You're Actually Looking For
The World Wars Night One Trial By Fire isn't a single thing you can just download from one place. It's a scenario set from a couple of different games and simulation projects, and I've seen people get confused about which one they actually want. Let me try to untangle it without making this longer than it needs to be. Most commonly, when people search for this, they're looking at one of two things: either a scenario pack for a tactical wargame like Paths of Glory or a similar WWI tabletop system, or it's connected to a WWII-focused miniatures project that uses "Night One" as its opening scenario name. There's also a notable indie digital game project that shares this title for its opening campaign mission. The name itself tells you what to expect. "Trial by fire" is a military term meaning your first combat experience. "Night One" means the opening night of the conflict. So you're looking at a scenario designed to simulate the initial shock and confusion of a sudden engagement during a world war. That's it. Nothing fancy.
Here's the practical part. If you're trying to get the scenario files or the game itself, your first stop should be the official forums for whichever system you're using. The designer or community usually posts updated versions there. I had a problem once where an older version of the scenario had a rule conflict with a rules supplement that came out six months later. The units in the night combat phase had overlapping visibility rules that made certain actions impossible to resolve correctly. I worked around it by noting which section of the night rules applied when the two documents disagreed, and I just went with the newer supplement's wording. It wasn't elegant but it worked fine for the game. If you're new to this, the main pitfall most people hit is assuming the scenario comes with all the necessary reference material. It doesn't. You'll need the base rules for whatever game system it's built for, and you'll probably need the night combat or urban combat supplement depending on the edition. Without those, you'll spend half your game time looking up basic mechanics instead of playing. Another thing beginners miss is the setup phase. These scenarios often have asymmetric starting conditions that aren't immediately obvious from the brief description. One side might have hidden deployment zones, or the other might start with reduced morale due to the element of surprise. I've seen multiple games fumbled because both players just set up their miniatures symmetrically like a normal battle, completely missing the order-of-magnitude difference the scenario was designed to create.
The down side of this type of scenario is that it tends to be swingy. The side that gets the first turn advantage in a night engagement can snowball pretty hard if they capitalize on it, and the disadvantaged side has a narrow window to stabilize. This is by design, but if you're looking for a balanced back-and-forth wargame, this probably isn't it. It's meant to feel chaotic and uncertain, which is historically accurate for what it's simulating. If you want something more evenly matched, look at the follow-on scenarios in the same campaign series instead. To actually get the files, check the publisher's website first. Most of these are distributed as PDFs or scenario sheets through their own storefronts or through platforms like DriveThruTables. The free versions usually have the core scenario but may skip the updated balancing patches. The paid versions typically include the full scenario package with errata and any subsequent rule clarifications. If someone is offering it as a direct download from a random forum post, it's likely an older unpatched version at best and a rehosted file with possible modifications at worst. I'd stick to official sources. The actual time commitment for running this one scenario is somewhere between two and four hours depending on how familiar you and your opponent are with the rule system. First time players should budget more. The setup alone can take twenty to thirty minutes if you're reading the deployment rules carefully, which you should be.
Get the Full Details

I don't have a specific download link to offer because the source depends entirely on which version and game system you're trying to run it under, and those change as updates come out. The official channels are the only place to make sure you're getting the current iteration.