***Undeservedly obscure atmospheric coastal horror*** Two couples in their 30s take a vacation to a secluded isle off the coast when a storm hits and people mysteriously start dying one-by-one. Kay, a troubled artist (Sarah Kendall), says she sees everything in her nightmares before it happens. I suppose “The Slayer” (1982), aka "Nightmare Island," could be classified as a slasher flick, but it’s more accurately a mystery-horror with haunting remote island ambiance, sort of like “The Shuttered Room” (1967), but mixed with elements of moody coastal horror, such as “The Fog” (1980). “I Still…