Grisly details of dєatᩚh row inmate Christa Pike’s sadistic murder of a classmate when she was just 18 have resurfaced in a new documentary by The Post.

Pike, 50, was set to be the first woman executed in Tennessee on Sept. 30 — but instead became the first prisoner in the US to survive lєthaℓ inje𝔠ti☉n.

She was sentenced to dєatᩚh for torturing and murdering Colleen Slemmer, 19, in a jealous rage in Knoxville in Janunary 1995 and then keeping a piece of her victim’s skull as a sick memento.

The details of the slaying continue to haunt those who worked on the case to this day.

Christa Pike is seen appearing in court for a hearing on Jan. 23, 1996. USA TODAY Network via Reuters Connect

“This case appeared in January of 1995, at the beginning of my career, and the nature of it has stayed with me throughout my entire career,” Knox County Forensic Anthropologist Dr. Murray Marks said on The Post’s new documentary “No Mercy: Christa Pike Unravelled.”

Slemmer suffered horrendous injuries during her 30-minute beating in a remote area of the University of Tennessee’s Knoxville campus where Pike and her 17-year-old boyfriend, Tadaryl Shipp had lured their victim.

Pike finished her off by bashing Slemmer’s head in with a large chunk of asphalt and then kept the piece of her skull as a souvenir.

Dr. Marks, who gave evidence during Pike’s trial, recounted the moment he was asked to testify about the missing piece of bone from the left side of Slemmer’s head.

University of Tennessee forensic anthropologist Dr. Murray Marks testifies about the wounds to Colleen A. Slemmer’s skull during Christa Pike’s murder trial on March 25, 1996. USA TODAY Network via Reuters Connect

“The District Attorney brought forth a little paper bag, and from that, withdrew a piece of bone, and so that was presented to me,” he said.

“I took it and, you know, I took my time and made sure it fit exactly.

“I looked at the edges, it fit precisely, the edges were a perfect match, and at that point, when the skull was demonstrated by me, the gallery gasped.”

Pike was handed the dєatᩚh penalty after her trial, while Shipp was given life in prison with the possibility of parole as he was underage at the time of the crime and not eligible to be put to dєatᩚh.

Former Knoxville News-Sentinel reporter Don Jacobs, who covered the case, revealed that Pike had written to Shipp to whine about her sentence.

“She did write to Mr. Shipp after she got the dєatᩚh penalty and she was apparently quite mystified why she got the dєatᩚh penalty,” Jacobs said.

A handout photo shows student Colleen Slemmer, who was tortured and brutally killed in 1995. Family handout

“She said, ‘I didn’t deserve that. I showed him a great deal of compassion because I didn’t let her die slowly.’”

An ambulance rushed Pike to hospital after she survived two doses of pentobarbital during her botched exє𝔠uti☉n at Riverbend Maximum Security Institution in Nashville.

She remained unconscious in the days after the bungled attempt until she woke up on Oct. 6.

She is currently being held at the Debra K. Johnson Rehabilitation Center (DJRC), formerly known as the Tennessee Prison for Women.

Pike’s lawyers said last week she is now speaking in full sentences.

Outgoing Tennessee Governor Bill Lee put a pause on all remaining executions this year following the fiasco and ordered an independent review .

The state’s prison boss, Frank Strada, resigned after the botched injections, saying it was “in the best interest of Tennesseans.”

Pike’s attorneys have demanded that her sentence be commuted, saying she has “endured punishment greater than any other person in the history of the American dєatᩚh penalty.”