A man who claimed the devil told him to kill a 12-year-old boy and drink his blood will be executed Wednesday — nearly two decades after he was sentenced to death for the senseless slaughter.

So-called “Vampire Killer” Pablo Lucio Vasquez will be executed after spending 18 years on death row in Texas for the 1998 murder, provided his lawyers’ last-ditch appeal fails.

Vasquez, 38, said he started hearing voices on his way back from a party to his home in Donna, Texas, telling him to kill David Cardenas, his cousin’s friend.

So he took the boy into a wooden shed, smashed his head with a pipe and cut his throat.

“Something just told me to drink,” Vasquez said in a videotaped confession.

Vasquez tried to cut off Cardenas’ head, but told investigators that “it couldn’t come off.”

Before leaving the bloody shed, Vasquez swiped a gold ring and necklace off his lifeless victim’s body and covered it up with aluminum sheets.

He pleaded guilty to murder charges and was sentenced to 35 years in prison in February 1999. Three others were also arrested in connection with the violent slaying — including his cousin Andy Chapa — which was originally believed to be part of a satanic ritual.

His lawyers maintain that he is mentally ill and therefore shouldn’t face the death penalty because he is incompetent, but Texas courts rejected that claim last month.

Vasquez will be the sixth person executed in Texas this year, and the 11th nationally.

With Post wires