Malacoda is the penultimate boss in Resident Evil: Revelations. It is a mass of monstrous parasitic worms created through the power of the t-Abyss virus. The worms in question are an infected form of a species in the Capsalidae subfamily, which is monogenean ectoparasites and so live out their whole lives in the same host.[note 1] The t-Abyss creations were named after "Malacoda", a character within Dante Alighieri's Inferno, who was the leader of the Malebranche: the twelve demons who guard Bolgia Five of Malebolge, the eighth circle of Hell.
When t-Abyss infects a host, any parasites in the body are infected as well. The unique mutations formed by the dual infections proved to be a great step in B.O.W. development.
A larval Malacoda is only about 1 cm in length and infects its host by secreting a liquid that contains the virus. The parasite mutates inside the host and matures rapidly. The synergistic effect of the t-Abyss and the chemical secretions causes an increase in the organism's metabolism, allowing it to grow in size. The size to which the Malacoda can develop depends on the size of the host - the bigger its host, the bigger it can grow. A researcher even noted down that a Malacoda would be capable of using a whale as its host.
A large group of Malacoda was seen swimming in the seas around the Queen Zenobia. They attacked BSAA agents Chris Redfield and Jessica Sherawat when they were attempting to board the Queen Zenobia, but they managed to repel them by using mounted guns on their boat.
When exploring the Zenobia's laboratory, Jill Valentine narrowly avoided death when a Malacoda parasite broke the aquarium glass, allowing the t-Abyss virus to flood the chamber. Fortunately, she had injected herself with a vaccine moments earlier, allowing her to avoid infection.
When Chris and Jill attempted to evacuate the Zenobia after the self-destruct countdown had been triggered, the Malacodas destroyed the rescue helicopter and revealed its host, a gigantic whale-like organism. With the help of rocket launchers dropped by Kirk Mathison, the agents managed to subdue the beast and boarded a second helicopter.
However, the whale revived itself and sent dozens of Malacodas after the two agents. They managed to weaken the host enough to fire a missile down its throat, killing it for good.
↑The Malcoda file in Revelations suffered a significant localization error whereby the family and subfamily were disregarded in favour of an unrelated species also within the Monopisthocotylea order, Wikipedia:Gyrodactylus salaris.