Who was the founder of the Ahmadiyya Muslim Community and what was his major claim?

The founder of the Ahmadiyya Muslim Community is Hazrat Mirza Ghulam Ahmad (on him be peace).

He claimed to be the Mujaddid (divine reformer) of the fourteenth Islamic century, the Promised Messiah (Second Coming of Christ), and the Mahdi awaited by the Muslims in the end days.

Hazrat Mirza Ghulam Ahmad Qadiani (as), The Promised Messiah and Imam Mahdi

His main objective and the purpose of his advent was to revive the teachings of Islam, to revive faith in one God and to unify the whole of mankind by bringing them all under the banner of the Holy Prophet of Islam (peace be upon him), who is the final messenger of God for the whole of mankind with a perfect and everlasting message for all times.