The Elden Ring: Shadow of the Erdtree DLC lets you turn into a Lamenter through a special item called the Lamentor's Mask. Here's how to get this item in-game.
Elden Ring Shadow Of The Erdtree – How To Get The Hidden Omen Form Of Laenter's Mask Location Guide
You can obtain the Lamenter's Mask from the Lamenter's Gaol dungeon, which is located northwest of Charo's Hidden Grave Region in the DLC. You specifically need to defeat the Lamenter, who is the boss of this dungeon, to unlock this item. You will need to travel from Jagged Peak to the Grand Altar of the Dragon Communion to reach Charo's Hidden Grave. Travel northwest from there and you will find the Lamenter's Gaol.
The Lamenter is a relatively easy boss that you can defeat with ease. He has a few jumping attacks, a head attack, and a special attack that spawns clones of himself to attack you with Black Flame (these can be defeated in one hit). Whether you are a magic or melee build, you should have no trouble taking on this boss. Most notably, he hits very hard, so make sure to duck away from his attack. After defeating him, you will receive the Lamenter's Mask.
The Lamenter's Mask turns you into a Lamenter while wearing it without any armor. It also gives a small boost to your damage negation. This form also has a special ability where re-equipping the mask causes your head to swell up and deal damage in the direction you're facing. However, it's not as effective against enemies with high balance. The only way to remove the Lamenter form is to die.
