Euler returned to St. Petersburg at the invitation of Catherine the Great. He was offered a generous salary and favorable conditions to resume his work at the Academy.

Euler's house was destroyed in a fire, and he narrowly escaped with his life. This event was a significant disruption, but he quickly resumed his work.

Shortly after the fire, Euler lost sight in his remaining eye, becoming completely blind. However, his mental acuity remained undiminished.

Despite his blindness, Euler continued to produce a remarkable volume of mathematical research. He dictated his ideas to his sons and assistants, who transcribed his work. His memory and mental calculation abilities were extraordinary.

Euler published "Institutiones calculi integralis", a comprehensive treatise on integral calculus, despite his failing eyesight. This work further solidified his central position in the development of calculus.

Euler died in St. Petersburg at the age of 76 after suffering a stroke. He was buried alongside his wife at the Smolensk Lutheran cemetery.