Lake Toba (Indonesian: Danau Toba) is a lake and supervolcano. The lake is 100 kilometres long and 30 kilometres wide, and 505 metres (1,666 ft) at its deepest point. Located in the middle of the northern part of the Indonesian island of Sumatra with a surface elevation of about 900 metres (2,953 ft), the lake stretches from 2.88°N 98.52°E to 2.35°N 99.1°E. It is the largest lake in Indonesia and the largest volcanic lake in the world. Lake Toba is the site of a supervolcanic eruption that occurred 69,000–77,000 years ago, a massive, climate-changing event. The eruption is believed to have had a VEI intensity of 8. It is believed to be the largest explosive eruption anywhere on Earth in the last 25 million years. According to the Toba catastrophe theory to which some anthropologists and archeologists subscribe, it had global consequences, killing most humans then alive and creating a population bottleneck in Central Eastern Africa and India that affected the genetic inheritance of all humans today. This theory however, has been largely debated as there is no evidence for any other animal decline or extinction, even in environmentally sensitive species. However, it has been accepted that the eruption of Toba led to a volcanic winter with a worldwide decline in temperatures between 3 to 5 °C (5 to 9 °F), and up to 15 °C (27.0 °F) in higher latitudes.

The Toba caldera complex in Northern Sumatra, Indonesia consists of four overlapping volcanic craters that adjoin the Sumatran "volcanic front". The youngest and fourth caldera is the world's largest Quaternary caldera (100 km (62 mi) by 30 km (19 mi)) and intersects the three older calderas. An estimate of 2,800 km³ (670 cu mi) of dense-rock equivalent pyroclastic material, known the Youngest Toba tuff, was blasted from the youngest caldera during one of the largest single explosive volcanic eruptions in geologic history. Following the "Youngest Toba tuff eruption", a typical resurgent dome formed within the new caldera, joining two half-domes separated by a longitudinal graben. There are at least four cones, four stratovolcanoes and three craters visible in the lake. The Tandukbenua cone on the NW edge of the caldera is relatively lacking in vegetation, suggesting a young age of only several hundred years. Also, the Pusubukit volcano on the south edge of the caldera is solfatarically active.

Though the year may never be precisely determined, the season can: only the summer monsoon could have deposited Toba ashfall in the South China Sea, implying that the eruption took place sometime during the northern summer. The eruption lasted perhaps two weeks, but the ensuing "volcanic winter" resulted in a decrease in average global temperatures by 3 to 3.5 degrees Celsius for several years. Greenland ice cores record a pulse of starkly reduced levels of organic carbon sequestration. Very few plants or animals in southeast Asia would have survived, and it is possible that the eruption caused a planet-wide die-off. There is some evidence, based on mitochondrial DNA, that the human race may have passed through a genetic bottleneck around this time, reducing genetic diversity below what would be expected from the age of the species. According to the Toba catastrophe theory proposed by Stanley H. Ambrose of the University of Illinois at Urbana-Champaign in 1998, human populations may have been reduced to only a few tens of thousands of individuals by the Toba eruption.

Pasai, also known as Samudera and Samudera-Pasai sometimes called Samudera Darussalam was a Muslim harbour kingdom on the north coast of Sumatra from the 13th to the 15th centuries CE. It was believed the word Samudera derived from Samudra meaning ocean in Sanskrit. According to Hikayat Raja-raja Pasai, it was said Merah Silu saw an ant as big as a cat, he caught it and ate it and he named the place Samandara. King Merah Silu later converted to Islam, known as Malik ul Salih, he was the sultan in year 1267 CE.Pasai exported its culture, and most importantly its language — an early form of Malay written in the Jawi alphabet — to a number of islands. Later, this language became the lingua franca among traders in what is now Indonesia and Malaysia.

Arab and Indian Muslims had traded in Indonesia and China for many centuries. A Muslim tombstone in eastern Java bears a date corresponding to 1082. But substantial evidence of Islam in Indonesia begins only in northern Sumatra at the end of the 13th century. Two small Muslim trading kingdoms existed by that time at Pasai and Peureulak or Perlak. A 1297 royal tomb at Samudra is inscribed entirely in Arabic. By the 15th century several harbour kingdoms developed, all ruled by local Muslim princes, from the north coast of Java and elsewhere to as far east as Ternate and Tidore in Maluku. Marco Polo spent five months here, he had Ferlec, Basma, and Samara (Samudera) mentioned in his travel story. Another famous traveller Ibn Battuta on his way to China stayed 15 days at Samudera.

The establishment of the first Muslim centres in Indonesia was probably a result of commercial circumstances. By the 13th century the collapse of Srivijayan power, drew foreign traders harbours on the northern Sumatran shores of the Bay of Bengal, safe from the pirate lairs at the southern end of the Strait of Malacca. Northern Sumatra had a hinterland rich in gold and forest produce, and pepper was being cultivated at the beginning of the 15th century. It was accessible to all the merchants of the archipelago who wanted to meet ships from the Indian Ocean. By the end of the 14th century, Samudra-Pasai had become a wealthy commercial centre, giving way in the early 15th century to the better protected harbour of Malacca on the south-west coast of the Malay Peninsula. Majapahit attacked and looted the place in the middle of the 14th century.

Pasai's economic and political power depended almost entirely on foreigners. Muslim traders and teachers probably participated in its administration from the beginning and were bound to introduce religious practices that made them feel at home. The first Muslim beachheads in Indonesia, especially Pasai, were to a considerable extent genuine Muslim creations that commanded the loyalty of the local population and encouraged scholarly activities. Similar new harbour kingdoms formed on the northern coast of Java. Tomé Pires, author of the Suma Oriental, writing not long after 1511, stresses the obscure ethnic origins of the founders of Cheribon, Demak, Japara, and Gresik. These Javanese coastal states served commerce with India and China and especially with Malacca, an importer of Javanese rice. The rulers of Malacca, despite their prestigious Srivijayan origin, accepted Islam precisely in order to attract Muslim and Javanese traders to their port.