Unstable Glass Soul

The Unstable Glass Soul is a usable item obtainable at the very start of the game by throwing the Key to the Heart into lava. It acts similar to the normal Glass Soul, but does not give a damage boost, and is used with left click instead of equipping it as an accessory. The player will also die when their health changes in any way, be it through getting hit, losing life through a debuff, or losing max health through any means.

Defeating Abaddon, Araghur, The Lost Siblings or Erazor in True Mode with the effect active will reward you with Glass Hearts just like with the regular Glass Soul. Additionally, dying with the effect active will cause a unique death screen to appear, which is covered in the sprite of the removed boss Challenger, also known in the community as "Noiseboi".

Trivia

 * This item is a reference to a modded gamemode of the same name for Hollow Knight, in which you permanently die when taking a single hit, just like the regular Glass Soul.