The Underworld (in Japanese: 魔界 Ma-Kai; "Fiend Realm") is one of the areas of Ninja Gaiden 2/Sigma 2 as well as Ninja Gaiden: Dragon Sword. This twisted, hellish netherworld is also the place where the Fiends live as well as where the Greater Fiends are sent to when they are dead. It is also known as the Realm of Chaos, the World of Chaos and the Chaos Realm.
Known as ""Hell" (in Japanese: 地獄, Jigoku) in Christianity, "Hades" (in Japanese: ハデス, Hadesu) in Greek lore, "Naraka" (in Japanese: ナラカ) in Buddhism, "Yomi" (in Japanese: ヨミ) in Shinto or simply the "Netherrealm" (in Japanese: ネサリアム, Nesariamu), this is the home of the wicked souls who die in the world of the living, and home of the most vile of creatures, the Fiends. Here Ryu Hayabusa, the last Dragon Ninja, must confront ghoulish beasts and devils from beyond his worse nightmares, some of them including Black Spider Ninjas that have perished at his hands.
- Infernal Forest (in Japanese: インファナル・フォレスト, Infanaru Foresuto):
- This hellish wilderness is here that Ryu Hayabusa confronts the first wave of newly reincarnated horrors, such as the Fiends and the fallen members of the Spider Ninja Clan. Among the magma filled land scape are withered trees engulfed in hellfire.
- Underworld Abyss (in Japanese: アンダーワールドアビス Andāwārudoabisu):
- One of Alexei's quareters in the Underworld, it is said that those who touch this frigid area can never leave and are doomed to never see the light of the Living again. Here Alexei had Sonia imprisoned to be his "pet" until Ryu dropped in and killed him.
- Hall of Blood (in Japanese: 血液のホール Ketsueki no Hōru):
- A gruesome path filled with pools of blood provided by veins surrounding the pillars. in some of the pools of blood are Fiendish Sharks who snatch up their unfortunate prey. Here Ryu does battle with a resurrected Genshin, now a Fiend, and the reincarnated Elizebet.
- Underworld Gate (in Japanese: アンダーワールドゲート Andāwārudogēto), also known as the "Gate to Hell" (in Japanese: 地獄の門 Jigoku no Mon):
- A large skeletal door that leads to the Abyssal Underworld. It is here where Ishtaros is fought, with Momiji held captive between two skeletal hands protruding from the Underworld Gate.
- Abyssal Underworld (in Japanese: 深淵のアンダーワールド, Shin'en no Andāwārudo):
- An area connected by narrow paths with large skeletal figures in the background. This is where Ryu has a confrontation with the Black Dragon Clone (if on Master Ninja Mode) and the skull-form of w:c:villains:Vigoor before plummeting down to the Edge of Darkness.
- Edge of Darkness (in Japanese: 復讐捜査線 Fukushū Sōsa-sen):
- A deep pit in where the battle with Nicchae and the final battle with the Dark Dragon takes place. After defeating the Dark Dragon, Momiji is rescued from within the pit and the two somehow manage to return home.
- One of the Underworld's names is actually that of the Netherrealm from the Mortal Kombat universe.
- The Realm of Chaos is also that of the distorted Chaosrealm from the Mortal Kombat fighting game franchise.
- The World of Chaos is the name of a massive and hideous Heartless monster from the first Kingdom Hearts videogame.