Dark Boo

Dark Boos are purple Boos that appear in Paper Mario: The Thousand-Year Door, Super Paper Mario and Mario & Luigi: Partners in Time. They may be found only in the hidden spot of the Poshley Sanctum and also in the Pit of 100 Trials. Dark Boos are more powerful than their white brethren. In Super Paper Mario, they hide in 3-D, waiting for somebody to walk by. Then, they flip to 2-D and attack.

Dark Boos reappear in Mario & Luigi: Partners in Time in the Gritzy Caves area. It is a faster and stronger type from a counterpart of a Boo. If a Dark Boo opens up its right eye, it means it is going to attack Mario by appearing behind Mario and ramming him. If a Dark Boo opens up its left eye, it means it is going to attack Luigi by appearing behind Luigi and ramming him. Additionally, if one of the Mario Bros. is attacked, the Dark Boo steals either a normal Item or a Bros. Item. In the next turn, the Dark Boo runs away after stealing the item. They are usually encountered in large groups, rarely appearing alongside other enemies.

Trivia

 * This is the only enemy that has appeared in both the Paper Mario series and the Mario & Luigi series, but not in the 'main' series.