Rollin said he set up a contest between them to decide who would make the poster.
Druillet won, of course but I suppose Devil made some projects which I'd love to see!
Well, in fact I think Devil never made any: in this interview
conducted in 1999, Jean said: "Le premier projet sérieux et fini qui est arrivé, c'était celui de Philippe Druillet. Et quand on l'a vu, on a dit : Ne cherchons pas plus loin, c'est l'affiche".
("The first serious and completed project that arrived was Philippe Druillet's. And when we saw it, we said: No need to look further, this is the poster")
BTW, I think it hasn't been mentioned here yet that Druillet collaborated to "Saga de Xam" (he actually made one black & white drawing)
for more information and an interesting interview on his work on "Le Viol du vampire".