Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
The element that destroys Demon Veen (in VN's novel Ada, 1969, Van's and Ada's father who perishes in a mysterious airplane disaster above the Pacific) is air:
Numbers and rows and series — the nightmare and malediction harrowing pure thought and pure time — seemed bent on mechanizing his mind. Three elements, fire, water, and air, destroyed, in that sequence, Marina, Lucette, and Demon. Terra waited. (3.1)