Ivan Porfiryevich Yelagin — favorite of Catherine II, a statesman, theater enthusiast, writer, and collector of ancient Russian literature, was also the head of Russian Freemasonry. He created a whole system of so-called Yelagin lodges. They were famous for their openness, and their meetings were accompanied by music. When Freemasons faced repression in the 1790s, Yelagin and his comrades continued to meet calmly.

On the island that belonged to him, now named after him, was the center of Russian Freemasonry. It is believed that Ivan Porfiryevich acquired the island specifically for practicing occult sciences. By the way, it was at Ivan Yelagin’s invitation that Count Cagliostro once visited St. Petersburg.

"The Pavilion under the Flag" is part of the palace and park ensemble, built according to Carlo Rossi’s design between 1812 and 1822 following the order of Emperor Alexander I for his mother, Empress Maria Feodorovna, widow of Paul I. Scholars suggest that Rossi erected the pavilion on the site of an old building constructed by order of one of the island’s owners, Catherine the Great’s Ober-Hofmeister Ivan Yelagin. It is believed that Empress Maria Feodorovna visited Yelagin Island every summer, and on each of her visits, the Russian flag was raised on the pavilion. Besides this pavilion, other famous works by Carlo Rossi are located on Yelagin Island — the Large Orangery, the Kitchen and Stable wings of the Yelagin Palace, and the Music Pavilion.

Freemasons’ meetings were held in the "Rotunda" pavilion (the pavilion under the flag), one of the buildings in the Yelagin Palace complex. During the visit of the famous magician and sorcerer Count Cagliostro, a ritual of initiation into the Egyptian-style Masonic lodge took place in the pavilion. Members of the lodge could be knights no younger than 50 years old or ladies over 35; frivolous youth were not admitted. A candidate for initiation had to endure a strict fast and undergo many rites. During the fast, the candidate took elixirs, pills, and drops from the Great Copt, as Cagliostro called himself. On a certain day of the fast, the novice underwent bloodletting and took a bath with a very potent substance, after which symptoms resembling mercury poisoning could appear. Those who completed the full course and repeated it half a century after initiation were promised by Cagliostro a lifespan of 5557 years.

The famous count’s visit is described in detail in Ivan Lukash’s book "Count Cagliostro on Yelagin Island," which is believed to be written from the words of the sister of Yelagin’s secretary. According to this story, Count Cagliostro conducted his experiments in a room surrounded by the four elements — air, water, fire, and earth.

According to one version, indirectly confirmed by finds in the basement of the "Pavilion under the Flag," where ceramic vessels and a brass knuckle duster were found, this pavilion was the place of Cagliostro’s "creations." The sunrise can be observed above it — the element of fire. The building is located at the confluence of the Neva River into the Malaya and Srednaya Nevka — the element of water. And earth and air are everywhere here.

During restoration work at the end of the 20th century, mysterious vessels of unknown purpose were found in the Rotunda’s basement. The Rotunda, or as it is also called, the Pavilion under the Flag, stands surrounded by larches, which are symbolic trees of Freemasonry, and is located at the meeting point of the four main elements — air, water, earth, and sun. It was no coincidence that Count Cagliostro highly valued this place, considering it the most suitable for mystical experiments. Here, through magical ceremonies and formulas, the Great Copt shared with his Russian colleagues his experience in controlling spirits, summoning shadows of the dead, and turning base metals into gold.

By the way, one legend says that when Cagliostro left Russia, he revealed that somewhere on the island a treasure was buried along with Solomon’s manuscripts and Homer’s sarcophagus.

Sources:

https://elaginpark.org/central-park/dvortsovo-parkovyy-ansambl/pavilony/

https://www.citywalls.ru/house7527.html

https://karpovka.com/2017-02-14/masonskij-sled-v-arhitekture-peterbu/