Mysteltainn

Mysteltainn is a succubus added in the collab "The Night Country and the Heroes of the Seven Worlds". She comes from ROBF4SU with Novissa, Tarsa, and her sisters Ogretooth and Executioner

Biography
She first appears at the end of chapter one with her two sisters as they are attacking Lady’s Village by plunging everyone into deep dreams. Lauratt and Rigeo meet them, and are aghast at their action.

Executioner proceeds to engage the duo in a fight, and, after a small skirmish decides that she must eliminate them. However, a small boy comes out from a building and surprise them. When Ogretooth tries to make a move, Magatsu steps in from nowhere and knock her back. Executioner and her decide to engage them in a fight, but Mysteltainn hold them back, and the trio leave.

They reappear in chapter 4 as they make their way to Izumo Village and discuss the "Queen of Dream"'s plan to bring in the "Land of Night". Ogretooth gets intercepted by Rovissa, and while the two fight, Executioner and Mysteltainn attack the villagers. After Mysteltainn transforms most women into nightmares with her chains, the trio leaves.

They show up once again during chapter 9, as Lest's party is stuck in the basement of the castle. With her two sisters, they chase the hero and his allies until they eventually end where Myusca, Vinum and Nemea are fighting Luka and Kazuya's party. She gets engaged in a fight by Alma Elma/Eden while Mysteltainn fight the heroes they were following and lose.

As the situation becomes critical for them, the fight slow down as the heroine accompanying Luka notices that both succubi trio are ready to jump at each other's throat. Mysteltainn and Vinum themselves recognize the issue, and back down with their own partners.

She shows up with her two sisters again in chapter 10 during the reunion between the members of the Alliance, and they make it clear that while they will help Est accomplish her plan, they have their own agendas.

They reappear again in chapter 12 and get in the way of the hero's group as the latter assault the Château des Romances, but are engaged either by Lilith, Astaroth and Morrigan or Eden, Zion and Gnosis depending on the route, leaving the path for the hero's party free.

They show up in the post-collab in Fairy’s Island. They reveal their true form and engage the party into a fight. After they've been beaten by the hero's team, they tell that they intend to settle down, but that they're looking for a weapon effective against angels as they risk being at war back in their world. To recruit them, Luka must bring them the "Immortal Trashing Whip shards" obtained by beating Daimyokai in the Snow Shrine during Musette's recruitment quest.

Monsterpedia entry
"A being who arrived from another world. Further Details are unknown."

Attacks

 * Attack
 * World-Ending Slash
 * Giga Slash
 * Dark Eternal
 * Drain Chain
 * Big Sister's Nightmare
 * Grand Sexcross
 * Kiss of Death
 * Demon Eyes of Silence
 * Demon Eyes of Petrification

Chapter 9
As the leader of the Cursed Sword Trio, she's extremely tough in no small thanks to her numerous status effects which include but are not limited to party-wide binds, silence and petrification. Curiously, she has a weakness of fire from out of nowhere, which Rovissa can easily exploit to end the fight fast before she decides to spam those status effects.

Real Fight
The trio of succubi can be dangerous thanks to its numerous powerful attacks, although they use unusually few Status Ailments. Keeping a protection against those is still preferable though. Blinding them to limit the number of attacks they land can be useful. All three succubi are weak to holy, making those attacks particularely effective against them. However, when it comes to actually taking them down one by one, each target has their pros and cons.

Executioner possesses both powerful and weak skills, and with a limited number of HP (300K to the ~500K her sisters have), it can be tempting to take her down first.

In the meantime, Ogretooth has strong attacks that can deal an impressive amount of damages, making her more dangerous on the long run.

Last, Mystletainn is the main source of Status Ailments with party wide binds, silence, petrification and death.