The Guerilla Union of Shrews in Mossflower or Guosim (pronounced [gaw-səm]) is an important group of shrews that traverse Mossflower Woods and the surrounding country. They are water creatures that typically use logboats on the many streams and tributaries for transportation. An argumentative and indecisive bunch, they are nevertheless deadly in battle and never unwilling to help Redwall Abbey's residents or other good creatures, and often act as transportation, crew or bodyguards for the protagonist. Their numbers vary by the book, ranging from 500 in Redwall to less than 40 in Marlfox.
They are always led by a Log-a-Log, a tough shrew who commands the bunch and is usually a bit more laid-back and less argumentative than the average shrew. The weapons of choice for the Guosim are small, deadly rapiers, they also use bows, and slings. They often wear brightly-colored headbands. They love to eat, and Guosim cooks are considered to be some of the best in Mossflower. Guosim are also often called rivermice by vermin. Guosim law states that if a Log-a-Log is murdered, his death must be avenged, and the shrew that avenges his death becomes the next Log-a-Log.
- The Guerilla Union of South Stream Shrews of Mossflower or Guosssom is a southern faction of the Guosim and is characterized by the usage of the Blackstone, which defines rank.
- The Guerilla Union of Roving and Fighting Shrews or Guoraf is another southern faction of the Guosim.
It should also be noted that in Redwall the tribe was portrayed differently than in later books; for instance, not only did the band have the name "Guosim", an individual shrew was named Guosim, but this may have been because she was "president of the shrew union", a title never referred to in future books, and the role as leader is now taken by a shrew titled Log-a-Log.
The Bladechant was a singing ritual that Log-a-Log Osbil and his tribe performed. It is a challenge for battle; after the Bladechant is performed, the Guosim under most circumstances must face their enemy. In Eulalia!, it is stated that if the Log-a-Log is unable to lead the charge, the rest of the Guosim cannot attack.
Not all Guosim tribes are known to perform this ritual.
The Guosim is based off of the Liverpool longshoreman's union membership. In the TV series, the Guosim are characterized as communists, and even speak in Russian accents.
The Guosim, or some form of it, appear in every Redwall novel with the exceptions of: