Sacrosanct was the homeworld of the Universal Church of Truth. Also known as G5 Deneb, it had been the Church's homeworld for sixteen years and was the Church's fifth homeworld in as many decades.
The cocoon in which Adam Warlock had placed Thanos had been found by the UCT who believed that it contain their messiah. The cocoon was kept on Sacrosanct until it was opened by Martyr and a resurrected Thanos was released from it. Soon after that resurrection, the planet had been depopulated of its billions of inhabitants, all of whom had either been killed by Thanos or had fled to escape him.