Baldur's Gate is full of tricky puzzles with hidden challenges, and completing some of them can be exceptionally challenging. The Self-Same Trial is a challenge you'll come across at the Gauntlet of Shar, and you must complete it to obtain an Umbral Gem. In this challenge, you will encounter counterparts of your party members, so be prepared for a fight. Here's how to complete the Self-Same Trial in BG3.
How to win the Self-Same Trial in BG3
The Self-Same Trial at the Gauntlet of Shar puts your party against itself by creating counterparts with all the same abilities. Note if you reduce your party count, you will fight less counterparts (e.g. two party members means two enemies).
I recommend using only Shadowheart and your custom character to enter this trial. It is best to ungroup your part, and keep the rest of the members outside the entrance for the Self-Same Trial. Enter the trial with two party members, and start fighting the counterparts to collect the Umbral Gem.
While it looks easy at start, there are certain conditions you must follow in the Same-Self Trial. For instance, attacking your teammate's counterpart will incur a heavy condition penalty that drastically reduces your abilities.
The fight will start with the counterparts located on a high ground, so you will already be at a disadvantage. You can always use dash or spells to fly up to remove the opponent's advantage, or change the terrain conditions for the counterparts.
Since the counterparts have all the same abilities, it can be a little frustrating. I relied a lot on ranged attacks to win this fight, personally.
One of the best methods is to figure out your party's weaknessess, and use the same attacks on the counterparts. Another is to avoid the counterpart's line of sight, and sneak up on them from behind—you can do this by hiding to clearly see the areas under observation, which almost always surprises the enemies, giving you an upper hand in the fight.
Self-Same Trail Location in BG3 (Map)
The Self-Same Trial is inside the second room, as located north of the Gauntlet of Shar waypoint (X: -828, Y: -728). To start the trial, perform a ritual at the sacrificial bowl (it is best to allow Shadowheart to perform it). Once that's done, walk inside with your party to face an equal number of counterpart enemies.
Best Party Members to use in the Self-Same Trial (BG3)
For party members, I recommend selecting Shadowheart, as these are Shar's trials, and Shadowheart is a devout worshiper of the Lady of Darkness.
Inside the arena you are bound to face a fight, so stick to characters that can resolve combat quickly with their skills. You can use your custom character to engage in this fight, but remember the counterpart will be equally strong and will have the same set of skills. I did this trial a few times with with different characters; it went best with Gale and Shadowheart, who allowed me to win the trial swiftly without taking massive damage.
How to get the Umbral Gem From the Self-Same Trial in BG3
The Umbral Gem — an essential item you'll need to traverse across the Gauntlet of Shar and in turn to the Inner Sanctum — will be dropped by the final counterpart enemy you defeat inside the Self-Same Trial arena. (There are a few other trials you must complete to collect the other Umbral Gems and activate the altar.)