Brynhildr in the Darkness OVA

It is mid-December, and SOS Brigade chief Haruhi Suzumiya announces that the Brigade is going to hold a Christmas party in their clubroom, with Japanese hotpot for dinner. The brigade members Kyon, Yuki Nagato, Mikuru Asahina and Itsuki Koizumi start preparing everything for the party, such as costumes and decorations. But a couple of days later, Kyon arrives at school only to find that Haruhi is missing. Not only that, but Mikuru claims she has never known Kyon before, Koizumi is also missing,

The "Abyss" is the last unexplored place in the world. Strange and wonderful creatures roam within, and it is full of precious relics that present humans can't recreate. Those that dare to explore the depths are known as "Cave Raiders." An orphan girl named Riko lives on the rim. Her dream is to become a Cave Raider like her mother and solve the mysteries of the cave system. One day, Riko starts exploring the caves and discovers a robot who resembles a human boy. (Source: Anime News Network, ed

Dawn of the Deep Soul continues the epic adventure of plucky Riko and Reg who are joined by their new friend Nanachi. Together they descend into the Abyss’ treacherous fifth layer, the Sea of Corpses, and encounter the mysterious Bondrewd, a legendary White Whistle whose shadow looms over Nanachi’s troubled past. Bondrewd is ingratiatingly hospitable, but the brave adventurers know things are not always as they seem in the enigmatic Abyss... (Source: Sentai Filmworks)

The second season of Made in Abyss. Directly after the events of Made in Abyss: Fukaki Tamashii no Reimei, the third installment of Made in Abyss covers the adventures of Reg, Riko, and Nanachi in the Sixth Layer, The Capital of the Unreturned. Note: The last episode aired with a runtime of ~48 minutes as opposed to the standard 24 minute long episode.

A murder occurs in an orderly world where robots are unable to kill humans. The robotic Europol investigator Gesicht takes the case, but the mystery deepens when he finds no trace of a human at the scene of the crime. As he pursues the truth, Gesicht uncovers the most evil manifestation of hate that history has ever seen, one that is bent on bringing destruction to the world ... (Source: Netflix)