“...I don’t like some of the people you’re hanging round with! I’m sorry, but I detest Avery and Mulciber! Mulciber! What do you see in him, Sev, he’s creepy! D’you know what he tried to do to Mary MacDonald the other day?”
-- Lily Evans to Severus Snape (DH33)
Mulciber was a student at Hogawrts at the same time as Snape, Lily Potter, and the Marauders. He was a supporter of Voldemort and became a Death Eater.
There are two Death Eaters named Mulciber. The first went to school with Voldemort and escorted him to Hogsmeade the night he applied to teach at Hogwarts (HBP20); the second a friend of Snape and Avery when he was in school some twenty years later (DH33). The earlier Muliber was an Imperius Curse specialist and had already been captured by the time Karkaroff came to trial — much to Karkaroff’s dismay, who was busy trying to trade Mulciber’s name for his own freedom (GF30). The younger Mulciber fought in the Battle of the Department of Mysteries (OP35).
- Mulciber who was Voldemort's friend in the 1950s.
- Mulciber who was Snape's friend in the 1970s.
Known for using the Imperius Curse on people, which would make him valuable to Voldemort.
Other canon notes and references
Rowling on Severus Snape and his fellow-Slytherin Mulciber at Hogwarts:
"Well, that is Snape’s tragedy. Given his time over again he would not have become a Death Eater, but like many insecure, vulnerable people (like Wormtail) he craved membership of something big and powerful, something impressive. He wanted Lily and he wanted Mulciber too. He never really understood Lily’s aversion; he was so blinded by his attraction to the dark side he thought she would find him impressive if he became a real Death Eater."
-- J.K. Rowling (BLC)
From mulceō (“I soften”), referring to a blacksmith heating metal to bend it.
Mulciber is another name of the Roman God Hephaestus or Vulcan, the god of "smelting" or "softening" metals, associated with hammers and fire. Interestingly Harry's cousin Dudley Dursley went to a secondary school called "Smeltings," possibly to lead him to a job in some mechanical field, since Vernon Dursley worked for a drill company. Perhaps the author is drawing a comparison between Mulciber and Dudley - a Death Eater and a bully.
Mulciber's name and the association with "melting iron" may be a reference to taking control of his victims by melting or softening their free-will with the Imperius Curse.
A fallen angel or demon named Mulciber was the architect of the capital of Hell - known as Pandaemonium - in John Milton's poem "Paradise Lost."
From the Web
"Mulciber (Riddle Era)" from Harry Potter Wiki
"Mulciber (Marauder Era)" from Harry Potter Wiki