The sorceror Ollerus the Unmerciful used an object known as the Warlock's Eye which allowed him to control the minds of his enemies to form a guerilla army, seeking to recommence the
Aesir-Vanir War and free
Vanaheim from
Aesir control.
The combined forces of
Asgard and Vanaheim battled him for 3 years before a small force selected for the stealth, including
Loki, were able to steal the Eye, freeing Ollerus's victims.