The 22-year-old was one of nearly 1,000 partygoers who attended the event, which was held at a rural farmhouse in Frenchtown Township, Mich. She was dressed as the Batman villain Poison Ivy, wearing black leggings, a purple wig and a leaf-covered top.
When Bruck never returned from the party, her family grew concerned and called the police, prompting a massive search for her.
Six months later, Bruck's body was discovered in a wooded area after police found her discarded costume nearly 15 miles away. It would be another year before her killer, Daniel Clay, was caught in July 2016. Clay was originally charged with second-degree murder, but a jury found him guilty of felony murder and concealing a body, for which he is now serving a life sentence.
Despite everything, Bruck's mother, Leannda Bruck, chose to forgive Clay.
"Today, with the strength of Jesus Christ I forgive Daniel Clay because if I don't all that has happened the last 2 and 3/4 years will destroy the rest of my life," she told reporters at the sentencing, per CBS News. "There are no winners in this, but by the grace of God, there will be no more losers."
Here's everything to know about the murder of Chelsea Bruck, including how her Halloween costume led to her killer's arrest.
Born on Jan. 28, 1992, to Leannda and Matt Bruck, Bruck was one of five children and was raised in the small town of Maybee, Mich. At the time of her disappearance, she was working as a hostess at a Monroe County restaurant called Olga's Kitchen, according to WXYZ-TV Detroit.
During a candlelight vigil held for Bruck just days after her body was found, her sister Kassandra described her as "so full of love," adding, "She was my best friend."
Per her obituary, Bruck loved the band Queen, the TV show Doctor Who and her pets. She also enjoyed baking, reading and playing video games.
On Oct. 26, 2014, Bruck attended a large Halloween party at a rural property in Frenchtown Township, Mich., dressed as the Batman character Poison Ivy. She was last seen leaving the party with an unidentified man around 3 a.m.
Her family reported her missing when she didn't come home, sparking a massive search effort across Monroe County and beyond. Kassandra told PEOPLE at the time that she knew her sister wouldn't have just left, leading the family to suspect something more nefarious had occurred.
"There is no way she just walked away on her own," she said. "We're a very close family, and we talk all the time. She was not unhappy or wanting to walk away ... She wouldn't go somewhere for this long without telling us."
The concerned sister added that the family would keep looking. "And we're not going to stop searching until we find her," she said.
In early April 2015, six months after her disappearance, authorities found part of Bruck's Poison Ivy costume at an abandoned industrial site in Flat Rock, Mich. Her remains were discovered about 15 miles away a few weeks later by construction workers.
A medical examiner later revealed that Bruck died of blunt force trauma to the head and had suffered numerous fractures to her face, jaw, neck and teeth, per CBS News.
More than a year after Bruck's body was found, police arrested a man named Daniel Clay on July 22, 2016. He confessed to killing Bruck right away, alleging that he accidentally choked her to death while they were having rough sex in his car.
Clay was originally charged with second-degree murder. However, according to The Monroe News, his charge was upped to open murder in November 2016, which allows a jury to determine if the crime was premeditated.
In May 2017, jurors found Clay guilty of both felony murder and concealment of a body. He was sentenced to life in prison without parole that July, per the Detroit Free Press. Monroe County Circuit Judge Daniel White called Clay a "liar, a rapist and a killer," before handing down the sentence.
One month later, Clay received an additional 40 to 75-year prison sentence for a separate sexual assault he committed shortly before his arrest, per The Monroe News. Clay allegedly forced his way into a Monroe County apartment and then sexually assaulted a woman.
The convict was 27 years old at the time of his arrest and had a lengthy criminal record dating back to 2007, including misdemeanor convictions for theft, assault and drug-related offenses. He has two sons with two different women, both of whom he owes thousands of dollars in unpaid child support, per WXYZ-TV Detroit.
He's now serving his life sentence, plus the additional sentence, at Macomb Correctional Facility in Lenox, Mich.
In May 2016, police collected DNA evidence from Clay after he was arrested on separate theft charges. When investigators ran it through the system, it came back as a match to DNA found on Bruck's Poison Ivy costume.
Bruck made her costume herself, sewing each vine on by hand and carefully piecing it together in the weeks leading up to the party, as her friends told WXYZ-TV Detroit.
During her testimony, Bruck's mom shared that she was very excited about her costume and hoped to win a prize, per WXYZ-TV Detroit.