Baldur's Gate 3 will test you in ways you never thought. You'll be tested on your wit, intelligence, perception, and more. One way this game tests you is through the many puzzles throughout the various areas. Some may be more difficult than others, but all are there for a purpose.
The Soft-Step Trial is one of those puzzles that is going to require patience, sacrifice, and dedication. Overall, it won't be the longest or most difficult puzzle you'll do in the game, but you will have to pay close attention as it can be a little confusing.
Location
You will encounter the Soft-Step Trial at the Gauntlet of Shar, coordinates (X: -830, Y: -754). To get here, you need to progress through the campaign. Once you reach the Underdark, you will be able to go to the Shadow-cursed Lands. Complete the puzzle that is located inside the Grand Mausoleum prior to going in the Gauntlet. You will be heading near the Treasure Room. After you go through the Bulky Door, you will begin the Soft Step Trial.
Soft-Step Trial
There will be many puzzles and trials throughout the Gauntlet of Shar. The Soft Step one will require you to have pretty good stealth as the whole point of it is to sneak around. You will be avoiding a shadow to get through a small maze. The trial should only take a couple of minutes tops if you are careful.
Sacrificial Bowl
The first step you must do is approach the Sacrificial Bowl just ahead of you against the wall. As with any trial, here you will choose to "Spill some of your blood in the bowl" as a sacrifice to this trial. This will take about 1 HP from you. Upon choosing this, your character will pull out a weapon and slice their arm, dripping their blood into the bowl. This will initiate the trial.
Avoid The Shadow
The two doors on either side of the sacrificial bowl will open. If you move your camera you can see over the wall. Go through the door to your left once you see the shadow moving on the other side. Utilizing your stealth, slowly follow this dark moving shadow on the floor. Experiencing stealth missions is vital in this game. Succeed in your perception checks to remain unseen.
You will want to hug the wall to your left and hide behind that wooden desk in front of you as the shadow will double back. As it moves past you and down the hall on your right, go around the wall you are hugging and to your left.
Head to but not through the archway in front of you opposite of the archway the shadow went in. Utilize Misty Step if you have it and bide your time. The shadow should come down that hall in front of you, stop in front of the archway just ahead and then turn around to head back. You will notice the red detection area in front of you that you must avoid.
Using the Misty Step, put yourself on the other side of that archway in front of you, so you're in that small room with the barred gate. If you don't have Misty Step, any invisibility will work though you'll have to move quickly if it only lasts for 10 turns. From here, quickly pull the lever then run towards the stairs on the other side of the wall next to you. Moving up the stairs, you will come to a gate with a shrine statue on the other side.
Once again, utilize Misty Step to get on the other side of the gate. Again, if you don't have Misty Step, there should be a Soft-Step Key on the table of the hidden room after you pull the lever. Keep an eye out for the traps that are hidden in this area. The door the key opens can also be lock-picked if it comes down to it. Do not go down the hallway that has a locked door as the key won't work on that door.
You can actually bypass this trial in its entirety by using Dimension Door if you have it. To use it, cast it through the left entrance. This will give you a line of sight where you can use a window to cast it through. You can have whoever the caster is in your party just teleport themselves then someone else that has Thieves' Tools proficiency lock pick the gate. Proficiency is important in a variety of areas for many missions in this game.
Approach the Umbral Gem and take it. Upon taking it, the torches around the room will illuminate and begin burning. Interact with the pedestal again and use the Umbral Transporter to transport you back to the Sacrificial Bowl. When you land, the torch above the Sacrificial Bowl will also begin burning, and you will be told that the first trial is completed. You may now exit back through the Bulky Door.
Reward
For completing this trial, you will receive the Umbral Gem. This gem is described as "Deep and dark, like a starless night" and it will have a cloudy purple haze to it. Gems in general can have a variety of uses. The Umbral Gem for example can help you progress by allowing you to use the elevator in the Gauntlet once you've completed the trials.