Cassandra Peterson, now 70, spent four decades building her iconic macabre persona into an international cult icon. Now, in “Yours Cruelly, Elvira: Memoirs of the Mistress of the Dark” (Hachette, out September 21) she details her trick-or-treat life in a candid memoir revealing her 19-year relationship with a woman — a female who she only identifies as ‘T’ — as well as encounters with some of the celebrity world’s biggest names — among them late basketball star Wilt Chamberlain, who she claims forced oral sex on her.“When a seven-foot-one, 300 pound man has his hand wrapped around your neck, there’s really not a lot you can do,” she writes of the athlete, who boasted in his 1991 memoir “A View from Above” that he had slept with 20,000 women.