Known as Hekatonkheires or Hecatonchires, they were giant beings from Greek myth. Having one hundred hands and fifty heads, their father Ouranos found them disgusting and threw them into Tartarus. Eventually, Zeus would free them and they would aid the gods against the Titans.
Hecatonchires can be encountered as a boss by using Kymopoleia's Hair Accessory in Shinjuku. The boss has a small chance of reflecting any physical attack and will always reflect fire. Rarely, he will drop an Abyss Seed. Turning this in to the Yagiya on Suginami Tunnels' gold-level instance's final floor will give players the ability to fuse Hecatonchires. He is fused through a special triple fusion of Ubelluris, Cyclops and Ogre.
Shin Megami Tensei IV Apocalypse
"Giants born from Uranus and Gaia. The name means "those with 100 arms." During the war of the Titans, Zeus brought them out of Hades. They helped him obtain victory."
—Shin Megami Tensei IV Apocalypse compendium
Hekatoncheires can be found in Camp Ichigaya. He can teach Nanashi the Panic Voice and Hades Blast skills through its Demon Whisper. It benefits from learning Physical and ailment skills.
Shin Megami Tensei V
"Giants born from Uranus and Gaia in Greek mythology. The name means "those with a hundred arms." During the war of the Titans, Zeus freed them from the underworld to help him obtain victory."
—Shin Megami Tensei V lore
Hecatoncheires can be found as a punishing foe in Akihabara. During a battle against the forces of chaos, Bethel's Greek division released a Hecatoncheires that did not discern between foes and allies alike, killing all in its path, including a Nordic squad led by a Valkyrie. The protagonist kills the Hecatoncheires per the Valkyrie's request as part of the quest "He of a Hundred Hands."
"Giants born from Uranus and Gaia. The name means "those with a hundred arms." During the war of the Titans, Zeus freed them from the Underworld to help him obtain victory."
—Persona 5 background
Hecatoncheires is the sixth Persona of the Hanged Man Arcana and can only be obtained through fusion in the Velvet Room. It is the first Persona usable by the protagonist in battle to learn the Regenerate 2 and Endure skills, and one of the four Personas to learn the Foul Breath and Fortified Moxy skills. When itemized through Electric Chair execution, Hecatoncheires yields an Endure skill card. Hecatoncheires is one of the Personas required to summon Chi You through advanced fusion.
Hecatoncheir is seen being used by Joker against Shadow Okumura. It has enough power to hurl his Execurobo aside and cripple it, allowing Haru to finish it using Milady.
Persona 5 Strikers
Hecatonchieres appears as a mini-boss in the Okinawa Jail, with the title "Hundred-Armed Watcher." After its defeat, it can only be obtained through fusion in the Velvet Room. A giant Hecatonchieres appears as an end-boss during the showdown in the Jail of the Abyss.
DemiKids Light & Dark
"An awesome demon with six arms that can crush the life out of any opponent."
Known as Hekaton, he can perform the combo Megaton with Krak or Cyclops.
Devil Children Messiah Riser
Hecatoncheir appears as the boss of Wind Makai. A general of the Imperium, he hides inside his cave while his soldiers guard the switches that will force him out. Unlike most bosses, he lacks a demon soldier to aid him in combat by healing or attacking foes. Upon defeat, he drops the Green Crystal.
Digital Devil Saga: Avatar Tuner 2 
Known as Hecatoncheires, he is fought as a boss is the second area of the game. After Serph and company climb a ladder with a small Karma Terminal near the end of the area, they will be confronted by a Karma Society officer, who will transform into Hecatoncheires and attack.
Hecatoncheires will counter physical attacks and has access to a special attack called Hundred Fist, which gets progressively stronger as he gets weaker and hits the whole party. Multiple party members with Tera and Dia will make the battle easier. Hecatoncheires can later be encountered as a normal enemy in the corrupted EGG Installation.
Shin Megami Tensei: Liberation Dx2
"Giants born from Uranus and Gaia. With 50 heads and 100 arms, their name means "those with 100 arms."During the war of Titans, Zeus freed them from Tartarus to help him obtain victory."