Step 1: Understand the word.
"Pernicious" originates from the Latin "perniciosus" meaning destructive or fatal. It describes something that has a harmful or deadly effect, often in a subtle or gradual way.
Step 2: Evaluate the options.
"Graceful" means elegant; "useful" means beneficial; "delightful" means pleasant -- all are opposite in meaning.
Step 3: Choose the synonym.
"Injurious" means causing injury or harm, which most closely mirrors the meaning of pernicious. \[ \boxed{\text{injurious}} \]