Mighty Nein

The Mighty Nein are an adventuring party and the protagonists of Critical Role in Campaign 2, starting with Chapter 9.

Meeting in Trostenwald
The seven individuals who would become the Mighty Nein first met in Trostenwald in 835 PD. Up to that point, they had been traveling in three groups:
 * Caleb and Nott helped each other break out of prison about five or six months prior, and had since been traveling and pulling cons together.
 * Jester and Fjord had met in Port Damali before running into Beauregard a few weeks later. They were heading toward the Soltryce Academy.
 * Mollymauk and Yasha were members of a carnival that had just arrived in town.

Caleb, Nott, Jester, Fjord, and Beau all happened to be staying at the Nestled Nook Inn, where Molly and Yasha came to advertise their carnival. Later, when the carnival turned suddenly violent, the seven of them worked together to defeat an undead menace and became embroiled in a local investigation. They formed an alliance to clear their own names, then chose to travel together afterward.

Yasha alone did not leave Trostenwald with the rest of the group, and was prone to disappearing on her own business for stretches of time. However, the group made a point of meeting up with her when they could. (Out of character, Yasha's absences were due to Ashley Johnson's frequent scheduling conflicts.)

Name
Upon defeating a gnoll pack that had raided the town of Alfield, the adventurers earned some renown for themselves. The local watchmaster, Bryce Feelid, asked for a group name for their Crownsguard paperwork. After conferring among themselves, the group settled on The Mighty Nein, a nod to Caleb's Zemnian accent.

From an out-of-character context, Liam O'Brien had been using German words and a German accent as a stand-in for Caleb's Zemnian. The rest of the cast picked up on this and would yell "Nein!", the German word for "no", whenever a nine was rolled or referenced. The joke became a running gag over several episodes. When asked to come up with a group name on the spot, nein was the first thing the group landed on, plus mighty for strength.

Trivia

 * The name may also be a reference to the 2016 video game "Mighty No. 9", whose voice cast included Matthew Mercer as Pyrogen (Mighty No. 1), Steve Blum as Countershade (Mighty No. 8), and Yuri Lowenthal as Beck (Mighty No. 9).
 * There are nine members of the adventuring party when Kiri and Caleb's unseen servant, Schmidt, are counted. Regardless, a more common interpretation counts Frumpkin and Jester's duplicate, making nine in total. The horse from Trostenwald can also be considered.
 * When the group teamed up with Kiri and Febron Keyes, making a group of nine humanoids, Caleb and Beau expressed regret that their name was no longer ironic.
 * None of the religious members of the Mighty Nein follow the approved deities of the Dwendalian Empire. Jester worships the Traveler (who may not be an outright deity), Molly worships the Moonweaver, and Yasha worships the Stormlord. Though Beau is nominally tied to the worship of The Knowing Mistress (a state-approved deity), Beau has expressed no interest or faith in her.
 * All of the Mighty Nein contrast heavily with the players' previous characters:
 * Fjord is very intelligent but one of the weaker members of the Mighty Nein, whilst Grog had low intelligence and was the muscle of Vox Machina. Also, as a Warlock, Fjord casts quite a bit of magic in addition to his melee combat, in contrast to Grog's complete lack of magic.
 * Beauregard is a self-admitted cynic with no qualms about hurting innocent people if it's necessary and adamantly refused to accept the Way of the Cobalt Soul until she was beaten into accepting. In contrast, Keyleth was an idealist who was traumatized when she accidentally killed an innocent child and happily set off on her Aramente. Both have poor social skills but Keyleth was socially awkward whilst Beau is just abrasive.
 * Caleb is a pure magic user while Vax was mostly a physical fighter. Vax made himself look presentable, while Caleb actively works to cover himself in dirt and mud. Vax spent his entire life with his sister, while Caleb's closest friend in the party is Nott who he only met comparatively recently. Vax regularly ran straight into danger with no problem risking his life for his friends, while Caleb always hangs back away from danger and considers himself to be a coward.
 * Nott is a huge contrast to Scanlan. Low charisma, awful singing, constantly nervous and jumpy where Scanlan projected confidence, and a goblin when Scanlan hated goblins for killing his mother (although Nott also hates goblins). Scanlan hated Trinket the bear, while Nott - as a pickpocket - loves trinkets. To top it off, Scanlan was a the oldest member of Vox Machina and a notorious womanizer while Nott is the youngest of the Mighty Nein and inexperienced in romance.
 * Vex was a smart and serious member of Vox Machina often coming up with plans and strategies while Jester acts ditzy and is a prankster. While Vex had difficulty trusting people growing up, Jester is an optimist who feels it's always better to trust someone even if they're obviously deceiving you. Jester is also less concerned about money than Vex was, having grown up being spoiled by her rich mother and having little understanding of how difficult earning coin was whereas Vex lived in the woods, worked hard for every coin she earned and was an expert haggler when it came to prices.
 * Mollymauk has only slightly above-average intelligence whereas Percy was the smartest member of Vox Machina. Percy placed importance on politeness, moderation and appearing noble whilst Mollymauk is ostentatious, shameless and revels in being chaotic. Molly is a melee fighter with some ranged spells whilst Percy was a ranged fighter with some melee capabilities. Percy sought revenge on those who harmed him and destroyed his family in the past; Molly has completely forgotten his past, permanently living in the moment and not wanting to know what happened. Molly is also genuinely nice despite coming off as untrustworthy, whereas Percy appeared straightforward but harbored a dark side and didn't always care about collateral damage.
 * Yasha is a Barbarian and pure physical brawler, a huge contrast to Pike's role as the group's healer. Yasha is also very closed off and blunt, where Pike was friendly and extremely kind. Yasha is one of the party's tallest and strongest members, Pike was a tiny gnome. Also, Pike used divine magic and worshiped Sarenrae the Everlight, whilst Yasha is a fallen aasimar which are a group whose inner light has been replaced by shadow.