Heliogabalus was a psychopathic Emperor of Rome. He was known for his wild parties, this being one of them. He decided to have a huge orgy and then to suffocate his guests with a cascade of rose petals.
Alma-Tadema was criticized for this painting mainly because it looked more like the guests were swimming in the rose petals instead of being suffocated. Alma-Tadema was working on this painting through the winter of 1887. To get the effect of the flowers just right, he sent for roses weekly from the Riviera during the four winter months.
Primary source: Victorian Painting by Lionel Lambourne, Phaidon, 1999, pg 294
