A Usutsuki Warashi is the malicious "cousin" of the Zashiki-Warashi and as such is a child-like ghost that manifests itself as a dirty, unclean spirit that spreads misfortune and misery in place of the usual happiness and fortune of the Zashiki-Warashi.
Usutsuki Warashi will emerge at night from dirt beneath floorboards or dark store-rooms and create creepy, disturbing noises, leave behind dirty footprints all over the home and induce feelings of dread in those around it.
Despite being a negative influence the Usutsuki Warashi still held a position much like the Zashiki-Warashi and thus attempts to outright banish the spirit would lead to disaster for the family involved.