Zassaliss, Harssacss, and Sesstra are major antagonists in the Redwall series book, Triss. They are a trio of sibling adders who have been joined together by their tails.
Zassaliss, Harssacss, and Sesstra were a trio of baby adders living in the abandoned fortress known as Brockhall. They lived there with their mother, Berussca, until their home is attacked by the ferret conqueror King Sarengo who is searching for the legendary treasure of Brockhall. Sarengo bounds the triplets together with his mace and chain and kills their mother before dying from their mother's poison. Zassaliss, being the biggest, takes the lead of the triplets and wears Sarengo's crown. Over time, their tails begin decomposing, leaving a terrible stench wherever they go. They learn to move as a unit and begin hunting for food around Mossflower Woods.
Their home is discovered, however, by two young creates who wandered off from their home, Redial Abbey. Upon hearing of Brockhall's discovery, the Redwallers begin to a way to locate in again. Once they learn about the snakes, however, their efforts shift to trying to rid them from Mossflower. They end up hiring a old blind red kite named Bluddbeak who used to be a renowned snake killer. Despite their efforts to dissuade him, Bluddbeak tries to fight the snakes anyway, resulting in him and his owl friend, Ovus, being murdered. Later, they trio attack and devour the pirate Plugg Firetail. This results in them being discovered by Princess Kurda and her minions. Kurds leads an attack on Brockhall, but are easily defeated by the snakes. Brachial is the attacked by the Redwallers who battle the snakes and kill them. Sesstra, however, is able to kill the otter Shogg, the titular character's best friend, before she dies.
- Despite being secondary antagonists, the adder triplets prove to be far more powerful and dangerous than Princess Kurda and Plugg Firetail.