Short answers, pulled from the story.
Leonhard Euler was a Swiss polymath born in Basel on the 15th of April 1707, active as a mathematician, physicist, astronomer, logician, geographer, music theorist and engineer. He founded graph theory and topology and is regarded as arguably the most prolific contributor in the history of mathematics and science, and the greatest mathematician of the 18th century.
Leonhard Euler introduced or popularized much of modern mathematical notation, including the concept of a function and the notation for a function applied to its argument. He used the letter e for the base of the natural logarithm, the letter i for the imaginary unit, the Greek capital sigma for summations, and popularized the Greek letter pi, which originated with William Jones.
Leonhard Euler spent most of his adult life in Saint Petersburg, Russia, and in Berlin, then the capital of Prussia. He arrived in Saint Petersburg in May 1727, lived twenty-five years in Berlin from 1741, and returned to Saint Petersburg in 1766, where he died in 1783.
In 1735 Leonhard Euler proved that it was not possible to follow a path crossing each of Konigsberg's seven bridges exactly once. The city sat on the Pregel River with two large islands connected by the bridges, and his solution is considered the first theorem of graph theory.
Yes, Leonhard Euler became almost blind in his right eye in 1738 and, after a cataract was discovered in his left eye in 1766, complications left him almost totally blind. His productivity barely suffered, and in 1775 he produced on average one mathematical paper per week with the aid of his scribes.
Leonhard Euler died of a brain hemorrhage in Saint Petersburg on the 18th of September 1783, after a lunch with his family, while discussing the newly discovered planet Uranus and its orbit with his student Anders Johan Lexell. The Marquis de Condorcet wrote that he ceased to calculate and to live.