- "It was a few seconds before Harry’s eyes discerned the building half-hidden amongst the tangle of trunks. It seemed to him a very strange location to choose for a house, or else an odd decision to leave the trees growing nearby, blocking all light and the view of the valley below. He wondered whether it was inhabited; its walls were mossy and so many tiles had fallen off the roof that the rafters were visible in places. Nettles grew all around it, their tips reaching the windows, which were tiny and thick with grime."
- —Harry Potter's impression of the Gaunt shack upon seeing Bob Ogden's memory of it in the Pensieve[src]
The shack was small, dilapidated, and thick with filth, but was somewhat kept in check by Merope. Morfin had nailed a dead snake to the front door of the shack. After Merope stopped taking care of the place due to her marriage and subsequent death, the house plunged even deeper into deterioration and declension.
Cecilia considered the shack and surrounding area an eyesore compared to the rest of the beautiful land owned by the Riddle family, and wanted it to be cleared away were it not inhabited by the Gaunts.
It was the home of Marvolo Gaunt and his two children, Morfin and Merope. They were the last remaining descendants of Salazar Slytherin and lived there in poverty, speaking Parseltongue to each other and living in deplorable conditions.
The Gaunt Shack and its inhabitants were once visited by the Ministry of Magic official Bob Ogden, who came to question Morfin about his habit of using magic in front of villagers. This is noticeably the first time Harry Potter, watching this memory in the Pensieve, sees Tom Riddle Sr..
Six months after Marvolo served his sentence, he returned to the shack in arrogant expectations that his daughter would be dutifully waiting for his return with a hot meal at the table. Instead, he found a clear inch of dust and her note of farewell. In another two and a half years later, Morfin returned and found his father dead and the family signet ring as the last heirloom for him to inherit.
About sixteen years later, when Tom Marvolo Riddle came to visit Marvolo to learn about his heritage, he instead found Morfin, along with a mass of filth, rusting pots, and mouldy food, which made the house nearly unrecognisable before Marvolo and Morfin's incarceration, due to Merope no longer taking care of the cleaning. After killing his father, Riddle framed Morfin for it, and left the Gaunt's house with Marvolo Gaunt's Ring. With that, the house became abandoned for years, and continued to deteriorate.
Horcrux hiding place
After the death of the Gaunt family the house was left deserted. Lord Voldemort decided to use this place to hide one of his Horcruxes, which was Gaunt's Ring taken from Morfin himself. The ring was left under the floor boards in the ruins of the house in a golden box with a curse of sorts placed upon it. In the summer of 1996, Albus Dumbledore visited the shack and retrieved the ring, reasoning that Voldemort would store his Horcruxes in locations relevant to him.
In 1998, Voldemort returned to the Shack to check on the Horcrux, after learning that one (Helga Hufflepuff's Cup) had been stolen from the Gringotts vault of Bellatrix Lestrange. Fearing for its loss, Voldemort traveled to the shack, as well as the Horcrux cave. To his fury, he discovered that the ring had been taken from the Shack.