For many seasons, Asmodeus terrorized Mossflower Woods, attacking and eating the woodlanders. However, he was somewhat of a legend, and very few knew if he existed. The young mouse warrior Matthias learned that years ago, Asmodeus had stolen the powerful Sword of Martin, and kept it with him in his cave. Matthias tracked down the snake and found his home; a huge sandstone quarry filled with tunnels. Matthias eventually found Asmodeus asleep, and quickly took back the sword. But the adder awoke and chased Matthias and his shrew friend Log-a-Log through his caverns, cornering them against a wall. Matthias was forced to fight, and he killed Asmodeus by stabbing him in the head several times after a great mental struggle against the snake's infamous hypnotic power.
Asmodeus continued to be a legend, and his descendants tormented Mossflower Woods for years to come.
- The name "Asmodeus" comes from the Biblical demon of wrath, Asmodai.
- In the first Redwall TV episode "Cluny the Scourge", Asmodeus was seen by an infant Matthias and Myrtle as they journeyed to Redwall Abbey. The sight caused Myrtle (who is seriously fatigued from the long journey) to run for her life and collapsed in the river. Matthias survived and was taken in by the Redwall dwellers, but Myrtle dies of exhaustion and was buried in the Redwall cemetery.
- As with other snakes in Redwall franchise, Asmodeus is a miniature counterpart to a dragon.