In the 12th century chronicle Gusta Danorum ("Deeds of the Danes") by Saxo Grammaticus one reads that the Curlanders and Estlanders aided the Swedes in the Battle of Bråvalla against the Danes who were joined by the Slavs, Livonians (a Finno-Ugric tribe that lived on the territory of today's eastern Latvia and Estonia) and seven thousand Saxons.
Saxo Grammaticus tells that the home of the best-known berserk of ancient Scandinavia - Starkadr/Starkather - was Estonia. In a later episode he mentions that Starkadr took part in looting raids to Couronia and Estonia.
Probably most known incident of Estonians (Oeselians from the island of Saaremaa) raiding Sweden is the attack of Sigtuna in the late 12th century. Sigtuna was the Swedish trading town on Lake Mälar, near Birka, which replaced Birka in importance in the later Viking Age. Oeselians and Karelians sailed to Sigtuna and occupied the town, which slowly lost its position of a commerce centre in favour to Kalmar, Stockholm, Uppsala and Visby on the island of Gotland.
The events were described in the King Erik's chronicle (Sweden, 14th century):
"Sweden then suffered serious harm
from the Karelians, causing great alarm.
They sailed into Lake Mälar from the sea
whether calm or stormy it might be,
secretly within the Svealand isles
in stealthily advancing files.
Once their minds to the idea did turn
that they the town of Sigtuna should burn,
and so thoroughly they put it to the flame
that it since then has never been the same.
There Archbishop Jon was killed,
a deed that many a heathen thrilled."
(source: Chronicle of Duke Erik: A Verse Epic from Medieval Sweden. Lund, SWE: Nordic Academic Press, 2012)
