The Borg was the master of the galaxy. It had assimilated every species in the Milky Way but one. That species was unknown. No member had ever returned from the unknown ones' planet.
The Borg approached the unknown ones' world with a million ships. They offered assimilation or destruction. Resistance is futile.
On the planet was a city with a single inhabitant. It was assimilated and the quintillion members of the Borg collective all descended into screaming madness.
Evolution had not equipped the Borg to share the mind of Cthulhu, who lives in not just three dimensions but in seven.