Little nightmare - OSRS Wiki

The Little Nightmare is a pet that can be obtained by defeating The Nightmare or Phosani's Nightmare. It is a much smaller version of her. The pet has a 1/800 drop rate in the original encounter, increasing by said amount for every additional player in the room when the fight started; 1/1600 in a duo, 1/2400 in a trio, 1/3200 in a 4-man and 1/4000 in a 5 or higher man encounter. At Phosani's Nightmare, the pet has a flat 1/1400 drop rate.

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I'm really enjoying this new birthday event item : r/2007scape