Gnomish
fecthir
noun. destroyer, slayer; bane
Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!
fecthir
noun. destroyer, slayer; bane
An archaic noun appearing as G. †fecthir “destroyer, slayer; bane” in the Gnomish Lexicon of the 1910s based on G. fectha- “destroy” (GL/34). Tolkien also had fecthir = fedron, but this was deleted.
Neo-Sindarin: Since I adapt the verb form as ᴺS. feitha- “destroy”, I would adapt the noun as ᴺS. feithor “destroyer”, but for “slayer, bane” I would use S. dagnir.