This feature used to exist in the game, but was removed. You can still see videos of the feature on YouTube, but keep in mind that this feature is not available as of now.
Summary
The Forgotten Soul, not to be confused with the Lost Soul , is a translucent red humanoid-like spirit that is notably known for being found near the location of one of the Old Gods, in the Ancient Underwater Cavern (Lava Version). It is noted that this entity is farmed for its experience points that it drops upon death (Essences).
The Forgotten Soul looks almost identical to the Lost Soul, with the only main difference is that the Forgotten Soul has a red hue, compared to the Lost Soul who has a blue hue in nature.
As of March 9, 2018, Forgotten Souls are dormant and do not attack the players, making them a resourceful strategy to farm and gain EXP (Essences). They drop around at least 5 Essences, also referred as Experience Points or EXP.
Trivia
- Due to its fast respawn time, it was a good way to farm EXP when under level 70.
- Its red hue is possibly a reference to the location they were found in, in a cavern under a lava or volcanic biome.
- The Forgotten Soul was removed along with the Lost Soul during the ??? Update