Aristarchus is an elderly Menaphite scholar in Pollnivneach. If you have completed Enakhra's Lament, he will provide some additional background on Enakhra and Akthanakos.
Aristarchus was not in Pollnivneach when it was added along with The Feud quest. He was later added along with the Enakhra's Lament quest, although he was not mentioned in the update.
Aristarchus is from Menaphos, the ancient capital of the Menaphite people. Whilst in the city, Aristarchus was a scholar in the grand Library of Menaphos, apparently studying the history of the Kharidian Desert. Although the length of his studies in the Library is unknown, his age and vast array of knowledge suggest he spent most of his life as a scholar.
Recently, Menaphos's neighbouring city of Sophanem was beset by a series of plagues caused by the desert goddess Amascut. When the plagues proved to be uncurable, Sophanem was put under quarantine to ensure that the plague did not spread. Menaphos, to further protect itself, was quarantined as well.
Aristarchus was not in Menaphos at the time, and was busy travelling, presumably on a research mission. When he returned to Menaphos, he found himself locked out, and was denied entry.
Today, Aristarchus has taken up residence in the small town of Pollnivneach, where he is biding his time while he awaits the hopeful reopening of Menaphos. His time is not spent idle, however, and he is doing further research in an effort to assist the Varrock Museum.
- This NPC is presumably named after the ancient Greek astronomer and mathematician, Aristarchus of Samos, who was the first known scholar to present a heliocentric model of the solar system.