Durarara!! x 18

Eighteen is a game app for iOS and Android featuring a roleplay narrative and puzzles. The game has collaborated with various anime series, including Bungo Stray Dogs, Kagerou Project, Re:Zero, Attack on Titan, and Durarara!! on two occasions. While the game is available in Japan and North America, the collaborations were only available in Japanese.

Setting
The story takes place in a mysterious dream world that exists in limbo between unconscious dreams and conscious reality, which appears as today's modern world. The dream world is divided into nine areas with more than 100 stages. Enemies wait on the stages and can be defeated by playing puzzles and using teammates' various abilities strategically.

Story
A disease called "Sleeping Beauty Syndrome" (Nemurihime Shoukougun) traps the hearts of young women who reject the realities of the modern world. These 'princesses' willingly become prisoners of dreams where their desires are realized. With mysterious clues from an anthropomorphic cat and a key in the number 18, players have to recruit a party and fight witches with the goal of saving the young women's hearts.

Gameplay
Inside the dream limbo, the hero meets an anthropomorphic cat who seems to know about the dream world. In the modern world, he is a professor who gives the hero vague clues and guidance, instructing the player to gather a team to rescue the women trapped inside the dream world. Players who 'dive' into dreams are called divers. The hero gathers other characters to make a party, and the party dives into dreams, where they use their various abilities to defeat the puzzles and lurking enemies.

Puzzles are played by connecting gems of the same color. Sliding through three or more gems of the same color will clear them, and six or more will convert the gems to a light orb, which can be used when an enemy attacks. Sliding through 18 or more gems will summon a Megami (witch), each of which possesses an exclusive skill. The game has no time limit, and the challenges presented by the puzzles and enemies increase as the levels increase.

The game has three modes: Quest mode, Dream Master mode, and Special mode. The Special mode is the one in which collaborations with various series takes place.

Durarara!! collaboration
The first Durarara!! collaboration took place from Friday, September 16, 2016, at 18:00 to Thursday, October 6, 2016, at 23:59. During this time period, several of the series' main characters were available as gacha divers and were able to be randomly acquired by players.

Durarara!!x2 re-collaboration
The collaboration quest and guerrilla event with Durarara!!x2 took place from Thursday, April 27, 2017, at 12:00 to Sunday, June 21, 2017, at 23:59, with a return of characters from the previous collaboration. Celty was available as a lottery prize bonus. Like the previous collaboration, it featured original stories blending Durarara!! with the story of Eighteen.

Resources

 * The official site for Eighteen explains collaboration bonuses in greater detail.
 * An article on the collaboration on a Japanese gaming site may provide further information.
 * Information and tips for a playthrough of Eighteen can be found here.
 * A partial transcript of the English version of Eighteen can be found here.