Union of Eight Guilds

Union of Eight Guilds is a coalition of various guilds hailed from the days of 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 in sheer numbers as an act of aggression. Unfortunately for them though, their attempt to dominate the dungeon ended in failure.

On the other hand, Momonga states that this union was considered to be the largest invading force to have ever been assembled as one in the game’s history.

In the Web Novel, they were believed to have met their final end in the Amphitheater. According to Momonga, it was "Nazarick's largest trump card secretly hidden away on Nazarick's 8th Floor which plays a huge role in defeating the union. The opposition viewed it as nothing more than a cheat to have. It was the reason why after 1500 people step foot into the Great Tomb of Nazarick and lost, they never once tried to regroup or challenge this dungeon ever again.

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 said allies. The formation of this union bolster a grand total of over 1500 players, and mercenary NPCs combined altogether within their rank and force. Among the invaders, there were players who possessed the World Guardian class.

In the Web Novel, one third of the 1500 invader were estimated to be at the max level of 100. The union is very strong in their own right as they managed to defeat all of the Floor Guardians on each of their respective floor.

When the intruders finally reached the 8th Floor, Aura states that only around 1000 to 1200 people are still able to survive or stay alive at that point. This meant that the previous seven floors have so far killed off members of the union in the range between 300 to 500.

Trivia

 * In the Web Novel, it was actually a union of six guilds rather than eight.
 * In contrast to this group, there exists an alliance of guilds who disbanded and then reform together as Trinity.
 * None of the top-tier YGGDRASIL guilds bother to participate during the invasion of the Great Tomb of Nazarick.