Rumil is one of the oldest known of the chroniclers, and appears already in the oldest texts. The meaning of the name is unknown, but it may be connected with the Noldorin word rum: "secret", "mystery".
Rumil was a Noldo and a sage living in Valinor in the city of Tirion. He was called the Elfsage of Valinor and the Ancient Sage of Tirion , and wrote many documents that especially concerns Valinor. Much of the Eldarin history science seems to have been based on his works. One of his most famous works is the Ainulindalë that tells of the Music of the Ainur and usually forms the introductory part of the Quenta Silmarillion.
Rumil seems to have abandoned his profession as a sage later (considering the unlikelyness of his death in Valinor), because in the many texts he is often referred to as a long-gone loremaster. There is a work called I Equessi Rumilo ("The Sayings of Rumil") that is a collection of his thoughts from the earliest days of the Eldar in Valinor. It treats, among other things, the Valarin language. The title might imply that he reached a status similar to that of Socrates, and was surrounded with disciples that wrote down his words (like Plato's Dialogues).
Something that signifies Rumil's greatness as a chronicler is that he in the Valian Year 1179 invented an alphabet: Rumil's Tengwar, properly called the Sarati. This is the oldest known alphabet in Arda, and was the one Fëanor was inspired by when he developed Fëanor's Tengwar, which was later used by almost all peoples in Middle-earth.