Qenya
morion
masculine name. the Dark One
toi aina, mána, meldielto - enga morion: talantie
they are holy, blessed, and beloved - save the dark one: he is fallen
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
Another name for Melko in the poem Fíriel’s Song from the 1930s (LR/72), possibly a combination of the stem form mori- of more “dark” and some variation of the agental suffix -ndo.
Conceptual Development: This name also appeared in the Qenya Lexicon from the 1910s, where it was glossed “Son of the Dark” (QL/62), a combination of the root ᴱ√MORO “dark” and some form of yondo “male descendant”.