Ys IX: Monstrum Nox (イースIX -Monstrum NOX-) is the ninth installment of Ys. It released in Japan for the PlayStation 4 on September 26, 2019; and February 6, 2020 in Asia.
On June 23, 2020, it was announced that the game would be released on PlayStation 4, Nintendo Switch, Steam and GOG in the west with the release window of 2021, published by NIS America.  The English localized PlayStation 4 version was released early February 2021, whereas the PC and Switch versions were announced for release on July 6th.
Following the opening of a teaser site featuring the first concept art, Nihon Falcom officially announced Ys IX: Monstrum Nox for PlayStation 4 during their investor's meeting on December 19, 2018. 
Connection with other games
- Old Seal Ring (STR/DEF +15, Critical Rate +30%) for Ys VIII: Lacrimosa of Dana
- Celcetan Wreath (DEF +10, HP +200, Resist Curse) for Ys: Memories of Celceta
Note: You don't need to complete these games, you only need to have the save of them on your system. It is currently unknown how the Switch version will handle the bonus for Ys: Memories of Celceta
What is real... is defined by what is not.
Who one is... is defined by who one isn't.
To move forward, one must accept these truths.
Yes, in this prison — this cradle — my dreams and reality became one.
~ Excerpt from Prison of Balduq
Ys IX: Monstrum Nox takes place in Balduq, northeast of Esteria in the Gllia region under Romun control. Balduq is known as 'the prison city' for its large prison facility inside the town and its architecture featuring high walls as a result of being built upon the fortified era. Furthermore the city is an important nexus for transportation in the region resulting in a wide range of inns, bars and trade centers. In more recent years public facilities such as theaters and baths are starting to be developed too.
In more recent years rumours of beings called "Monstrum" are said to have started appearing creating a huge fuss with its residents. The Monstrum are said to possess unique supernatural abilities, some of them seem to be helping the poor and helpless, others are outlaws up to no good. This dichotomy results in the villagers becoming scared/wary of them.
Balduq Prison, located at the edge of the city, supposedly is the largest prison facility of the Romun empire. Originally it was a fortress built to protect the Romun border, it is more recently that it has been repurposed as a prison facility. Featuring only one entrance through a drawbridge and surrounded by steep rock walls and deep moats it is said to be inescapable. The townsfolk does not hold the prison facility in high regard as the facility also is used to detain critics of the empire as does it detain political prisoners.
- Adol Christin, the game's main protagonist who is falsely accused of being the prime suspect behind the disappearance of a Romun military fleet during a voyage in the Atlas Ocean. With the aid of the mysterious, cloaked female and his new disguise, Adol works to break free from his imprisonment. He can make use of a red and black trajectory that is released from his left hand to traverse to various locations as well as in battle. During the course of the game, he gets caught up in the strange incident involving the cryptic Prison City and the Monstrum.
- Dogi, Adol's best friend, and travel companion. Dogi is a man with a huge body and power that can break through any wall. He's been through many various adventures with Adol and can now react calmly to any inhuman activity or strange events. After departing Altago and arriving in a Romun city, he is found with Adol and taken under suspicion.
- White Cat, a Monstrum active in the prison city as a bandit known for stealing from the rich to redistribute it for the poor. This character seems to have good contacts with residents and shows off light and flashy performances throughout the city. It specializes in speedy attacks that make use of its supple body like that of a cat.
- Hawk, a Monstrum who calls himself “Heaven’s Justice” and murders in the city streets. Not only does he have a bad drinking habit, not just getting in fights with battle enthusiasts, he also goes out of his way to create trouble on his own. It is said that he is the least cooperative of the Monstrum appearing in Balduq and even other Monstrum is having trouble with his behavior.
- Raging Bull, a Monstrum who has an upbeat personality and is known as the "mood maker" of the group. She is strong but has plenty of cute qualities about her as well. With the use of her weapon, a war hammer, Bull lacks speed but makes up for it by swinging it around to hit enemies all around her. There are rumors that she may have blood relatives living within the wall of Prison City, but this claim has yet to be proven.
- Doll, a Monstrum who's true identity and origins are unknown. Her beautiful, doll-like appearance (hence the name) was created by a master craftsman. To the other Monstrum and monsters of Balduq, she seems very mysterious and her peculiar movements are difficult for humans to follow in battle.
- Renegade, a Monstrum who has an androgynous image, with long, silvery hair and bearing a worn and battered coat. He is humble in appearance and is rarely seen by the village folk of Balduq, due to his strong and mature temperament, despite being so young. It is believed that he has some knowledge about the history of the Gllia region and the Prison City itself.