Dulak is an evident alternative to the usual incantation of Nox. It is used to extinguish the spell Lumos (canon) just as Nox is, though the fact that Harry uses Dulak instead may be because of his interest in Science Fiction and Fantasy.

"Harry extinguishes his light with the word dulak; the typical word for extinguishing light is nox, Harry instead used the word the Raistlin Majere used to darken the Staff of Magius in the Dragonlance series." - TV Tropes