The Mamba du Mal is "one of the deadliest snakes in the hemisphere," and kills its victims by strangling them, leaving dark purple bruises.
The snake is also capable of communicating, and was trained by VFD to communicate vital messages as evidenced by the following excerpt from The Mamba du Mal: A Snake That Will Never Kill Me by Tony "Mommy" Eggmonteror. The snake can recite certain phrases in English, such as "summer is" (enemies are nearby), "over and gone" (probably in disguise0 or "dying" (beware of arson).
Count Olaf used the Mamba du Mal's venom to kill Monty Montgomery. However, while the injection site looked like a snake bite, Klaus Baudelaire noticed that Monty did not have any bruises. This proved that the Mamba du Mal was not responsible for Monty's death.
The Mamba du Mal is killed by unknown means, and its body (which is now framed) is auctioned off to the "Esmé Squalor Fan Club" at the In Auction. The Fan Club are known as associates of Count Olaf.
- The Mamba du Mal never appeared in the 2004 film. Its role of the accused murderer of Dr. Montgomery was instead played by the Incredibly Deadly Viper. It is possible that it is in Uncle Monty's house somewhere amid all the reptiles.
- The Mamba du Mal is mentioned in the Netflix adaptation. Olaf uses its venom to kill Monty, but still blames the Incredibly Deadly Viper as Monty's killer like the film. When Olaf talks about the Mamba du Mal, the scene shifts to a snake, which may be the Mamba du Mal.