Union of Eight Guilds

Union of Eight Guilds is a coalition of various guilds in YGGDRASIL.

Background
Just as the name suggests, it is a union made between eight unknown guilds who banded together under a common cause. While it's unclear what their objective was supposed to be, they did tried to conquer the Great Tomb of Nazarick once through invading it. Unfortunately for them though, their attempt to dominate the dungeon ended in failure.

Momonga states that this union was considered the largest invading force to have ever been assembled in the game’s history.

The Undead King Arc
Momonga recalled his experience of the time when there used to be an alliance of guilds who plotted a large-scale invasion against the Great Tomb of Nazarick and miserably failed in doing so.

The Bloody Valkyrie Arc
According to Sebas Tian, in the past, the invaders of Nazarick were able to breach the defenses of the 7th Floor. However, they are unable to overcome the 8th Floor where Victim and Aureole Omega are resided in and died there.

Strength
According to Momonga, there were eight guilds which join together with their allies. The formation of this union bolster a grand total of over 1500 players, and mercenary NPCs combined as part of their rank and force.

Trivia

 * In the Web Novel, it was actually a union of six guilds rather than eight.
 * Let alone this group, there also exists an alliance of former guilds who now called themselves, Trinity.
 * None of the top-tier YGGDRASIL guilds bother to participate during the invasion of the Great Tomb of Nazarick.