H.P. Lovecraft’s legacy outlives the books he's written: an entire subgenre of fiction is dedicated to his name. Lovecraftian horror is described as a subset of cosmic horror that emphasizes the fear of the unknown, something Lovecraft was certainly known for implementing in his spine-tingling tales.
Whether you recognize the image of Cthulhu or other Eldritch horrors, Lovecraft's influences have subconsciously pervaded popular culture to its core. With cosmically significant figures appearing all over like the race of Engineers and monstrous black goo in 2012's Prometheus and 2017's Alien: Covenant (or that godforsaken baby-faced sun abomination on Teletubbies), and subtler Lovecraftian nods like insanity meters in videogames and appearances of Cthulhu or Yog-Sogthoth on television, his works couldn't be more apparent nor important in the 21st century.
Indeed, Mr. Lovecraft has affected the development of literary horror (and more) beyond face value; many concepts like forbidden knowledge and cosmic insignificance have permeated works since his time, and continue to be apparent today, having been thematically important to generations of authors and creators.
In case you don't know some of them, here are just a few figures influenced by the master of Eldritch horror and his expansive works.