[Explanation of some stuff, read after ep4 maybe]

Bal-Masque essentially has 1 main leader and 3 assistants:

Leader - "Snake of the Festival" (Name unknown)

Assistants:
1. Strategist - "Arbiter of Reverse Reasoning" Bel-Peol
2. General - "Metamorphosis" Sydonay
3. Priestess - "Master Throne" Hecate

In a conversation with Yuji in volume 1, Alastor mentioned that long ago, the "Snake of the Festival" was "vanquished" by Flame Hazes, but we don't hear anything of that afterwards.

For centuries, the three Assistants have been striving to achieve their "Great Destiny", and the first part involves bringing their Leader back.

In order to do so, they prepared an armour to act as a "temporary body" for their master to reside in. (I still haven't figured out how they summoned his existence back exactly, this might mean the Flame Hazes weren't thorough when "eliminating" his existence completely. )

This body, titled "Tyrant I", is in the shape of a medieval armour placed in a crucified position in Bal-Masque HQ. Though their master is not fully self-aware and thus Tyrant I is still a mindless automaton.

While we know Margery's hated nemisis, "Silver", looks exactly like this armour, whether "Silver" is captured by Bal-Masque, or whether "Silver" is really "Snake" is unknown.

However, that was just the first part of "Phase I", as they still needed to get their master to become fully sentient again, but they could not complete their plan without a proper instrument. It wasn't until they heard of Pheles and her ultimate creation, the Reiji Maigo, that they see the perfect tool to execute this plan.

Hence, Bel-Peol hired Sabrac, the assassin-for-hire, to pursue Pheles and Johann everywhere, and tasked him with 2 objectives: 1. capture the Reiji Maigo, 2. insert a Manipulation Spell related to the "Great Destiny" into the Reiji Maigo. With objective 2 more important in case objective 1 could not be achieved.

This insertion Spell allows in a second silvery armour, titled "Tyrant II", to reside inside the Reiji Maigo. I'll get to the connection between the 2 Tyrants below.

As we all know, Sabrac finally succeeded in mortally wounding Johann. In order to save Johann, Pheles shoved Johann's existence into the Reiji Maigo and forced it to randomly transfer before quickly retreating. Sabrac managed to insert the Spell into the Reiji Maigo just before it disappeared, as witnessed by Wilhelmina, who was also injured during the ambush and escaped afterwards. This happened sometime just before volume 1, when the human Sakai Yuji was turned into a Torch as he was consumed by the "Hunter" Friagne. By chance, Yuji became a special Torch, a Mystes, and again by chance, the Reiji Maigo was transferred to him of all Mysteses.

As long as objective 2 was achieved, Bal-Masque was content to wait for centuries and bide their time until they could find the Reiji Maigo again.

During all this time, Sydonay was moonlighting as a bodyguard-for-hire, since he was too bored with simply "waiting". By luck, he stumbled into Yuji while being bodyguard to Tyriel and Sorath. Excited, Sydonay returns to Bal-Masque to report on the news. (Volume 3 & 4)

Once Bal-Masque is finally able to pin-point the location of the Reiji Maigo, they can begin to "link-up" the 2 "Tyrant" armours and begin "transmitting data". Here comes the crucial part: The "data" being transmitted from "Tyrant II" inside Yuji to "Tyrant I" inside Bal-Masque HQ is none other than Yuji's own emotions and feelings. This was the key to getting "Snake" fully sentient once more. Hiring the mad scientist Dantalion helped sped up the transmission process considerably. (Volume 6 & 7)

At two separate occasions, Wilhemina and Pheles came very close to derailing their plan though.

While Wilhelmina does not know about the plan, she knew at least the Spell wasn't something good, and sought to destroy Yuji initially, but was dissuaded from doing so by Shana. (Volume 8 & 9)

And Pheles, obsessed with reuniting with Johann, unwittingly awoke "Tyrant II" prematurely, causing "Tyrant I" back in Bal-Masque HQ to become unstable. Hecate had to personally come out to shove "Tyrant II" back inside Yuji. (Volume 12 & 13)

(While being inside the Reiji Maigo all this time, Johann apparently seems to be aware of the "Tyrants" and Bal-Masque's plot, and briefly took over Yuji's existence to warn Pheles about it. Pheles has yet to divulge what he told her)

After that incident, Yuji decided to beef himself up. During this time, he began having visions of someone speaking to him through a mirror. But he usually dismissed them as dreams.

Then came Christmas time, when Shana and Yoshida decided to set up an ultimate show-down and force Yuji to decide who he really loves. Before he could do so, Sabrac strikes again.

While eliminating Shana, Margery and Wilhemina is a bonus, Bel-Peol actually tasked Sabrac with another objective, that is to place a key-like Hougu into Yuji's pocket.

As in the anime, Sabrac barely escaped the fight with Shana with his life, but he succeeded in his main objective once again.

After "cleaning up" the city after the battle, Shana and Yoshida went back to their waiting places (as seen in the last few episodes of the second season), and have Yuji choose who to go to.

As Yuji stood alone while the others left, he reflected back on his experiences ever since becoming a Mystes. And he makes his choice.

"SO, YOU HAVE MADE YOUR CHOICE, SAKAI YUJI." Boomed a voice in his head. (This sent me chills as I read that.  )

He then noticed the Key in his pocket, which triggers the "voice" he's been constantly hearing in his head to call out to Yuji, commending Yuji as a most "worthy human host" for his "regal presence". As though being hypnotized, Yuji slowly walks off the rooftop of a building, and then vanishes without a trace.

Back in Bal-Masque HQ, everyone was eagerly expecting someone to turn up, and then someone apparated out of nowhere, crushing the "Tyrant I" armour that lay there.

Bel-Peol then proclaimed the return of their leader: "Snake of the Festival" Sakai Yuji. (I went ZOMG when I read that  )

Bel-Peol later explains to Sabrac that while "Snake" can do away with the Torch that is Yuji now that he's returned and take up his own human form, "Snake" decided to continue possessing Yuji's body (much like how Rami possesses a Torch), because he finds Yuji "interesting".