From Advent of Ascension Wiki
This creature is a Hunter mob that can only be damaged when the player reaches a certain Hunter level.
Make sure to check the level requirements carefully before making rash decisions.
Nightmare Spiders are aggressive mobs that spawn in Haunted Houses located in Mysterium. It is a melee mob that blinds (4 seconds on hit) and nauseates (0.5 seconds on hit) players. It grants 60 Hunter experience points when killed. It is advised to stay out of melee range of this mob. The Nightmare Spider is a hunter mob that can only be damaged if the player is level 20 or higher.