Ali ibn Abi Talib (RA) — 4th Caliph
656–661 CE · The Rightly-Guided Caliphs
Ali, the cousin and son-in-law of the Prophet ﷺ, becomes Caliph amid civil conflict (the First Fitna) including the Battle of the Camel and the Battle of Siffin. Despite political turmoil, Ali (RA) is known for extraordinary knowledge — the Prophet ﷺ said: "I am the city of knowledge and Ali is its gate." He is martyred in 661 CE, ending the era of the Rightly-Guided Caliphs.