The monster who ambushed his estranged girlfriend in her Harlem apartment building and stabbed her as many as 50 times claimed he was blackout drunk and can’t remember, prosecutors said Wednesday.

Luis Marquez, 41, was arraigned on murder charges and ordered held without bail in the vicious early Monday morning attack on Carmen Lopez, his on-again, off-again girlfriend of five years — as disturbing new details in the blood-curdling attack emerged, prosecutors said.

Marquez allegedly stalked Lopez during her night out with friends at a local club, then lay in wait for more than two hours in the stairwell of her West 129th Street apartment until she returned home around 4 a.m., Manhattan Assistant District Attorney Jonathan Junig said in court.

“When the victim exited the elevator ont

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