Mōryōmaru

"It was a mistake for Naraku to release his heart that day. He even entrusted the Fuyōheki to the Infant, to protect his heart. That's when the Infant had an idea, to create a durable armor of his own. And that armor is Mōryōmaru, who ate the dead Goryōmaru."

- Hakudōshi.

is a composite demon vessel created by Hakudōshi and used by the Infant to combat Naraku.

As far as personal essences go, he is actually the Infant, the physical embodiment of Naraku's heart, and his yōki cannot be detected due to the Fuyōheki, a stone which completely masks the presence of any yōki near it.

Overview
When the Infant decided to replace Naraku, he needed an armor to protect himself with. To this end, Hakudōshi slew a soul-eating demon for the haku (life force) in its belly. He tried patching together the corpses of demons to create prototypes of the armor, but each ended in failure as the haku belonged to humans and the patch-work, malformed bodies were rejected by the haku. Eventually, Hakudōshi discovered that if the form was humanoid, the haku would remain, thus Mōryōmaru was created after several more prototypes.

Mōryōmaru fought against Kōga for his shards, trying to eat him; however, the arrival of Inuyasha's group forced a change in plans, with Hakudōshi redirecting Mōryōmaru's attention to eating Inuyasha. However, the battle didn't go well, and had Mōryōmaru in danger of being destroyed. Because of a telepathic message from the Infant, Hakudōshi protected Mōryōmaru from the Adamant Barrage, using him to steal the shards of demonic energy from Inuyasha's group, to keep the location of the Fuyōheki safe.

Given to Kanna and the Infant by Hakudōshi, Mōryōmaru was used to attack Goryōmaru's temple in an attempt to eat him to gain an increase in strength; however, after succeeding in devouring the priest's right arm, Mōryōmaru was sealed and used as a replacement arm. Goryōmaru made use of Mōryōmaru's ability to shot demons out of his arm as beams of light, creating Goryo urns for his apprentices to slay demons with. Hakudōshi later beheads Goryōmaru, allowing Mōryōmaru to be freed and finish devouring the priest.

The Infant later places himself in Mōryōmaru, but Naraku manages to lock him inside a cell with statues created by Goryōmaru. He persuaded Kagura into releasing him by promising her the freedom she sought, gaining her assistance in his plan to replace Naraku. He gives her Goryo urns for bandits to use as a distraction for Inuyasha's group. With this occurring, Moryomaru could absorb more demons to increase his strength.

When Kagura betrays him and Hakudōshi by allowing Kohaku, who she was supposed to steal a shard from to increase Mōryōmaru's powers, escape, Mōryōmaru reveals himself to Inuyasha's group. Hakudōshi gives him the shard stolen from Inuyasha's group and he takes off to get Kohaku. He captures Sango, who also chased after him, using her as bait to lure out Kohaku. Using the Infant's ability to read hearts, Mōryōmaru realizes Kohaku regained his memories as they boy decapitates him. He nearly succeeds in taking the shard, only to be stopped by the arrival of the rest of Inuyasha's group. Once they reveal Kagura told them the Infant is inside him, Mōryōmaru is forced to flee on its orders to get stronger so it can remain safe inside him.

Increasing Powers
As the story progresses, Mōryōmaru begins to increase more and more in power as he absorbs different kinds of yōkai to gain their abilities. He seems to be more adept at this than even Naraku as he sends pieces of his flesh out to gather demon parts. Once he sent his arm to kill a bunch of snake humanoid demons and then absorb them for their vemon, which Kōga witnessed. Here the main focus of the story begins to shift from finding and defeating Naraku, to stopping and destroying the growing Mōryōmaru. Like Naraku, Mōryōmaru rarely appears before his enemies; however, instead of trickery, he often uses other powerful yōkai (whom he plans to devour later) that he influences to fight them. A piece of his flesh is eaten by a venomous demon, allowing to grow in size and power; at this time, the Dragon- scaled Tessaiga proved to be a very useful weapon against the demon and, possibly, even Mōryōmaru himself. Even Naraku believed that Mōryōmaru faced danger from the blade.

After he revives the corpse of Meiōjū (a turtle-like yōkai) and it regains its armored shell, Mōryōmaru takes control of it and allows Inuyasha to strike Meiōjū with Kongōsōha (Diamond Spear Blast). Though Sesshomaru was trailing Meioju back to him, Naraku surprsingly ordered Byakuya to create a distraction to allow Moryomaru absorb Meioju. Once he's done so, Moryomaru gains a powerful armored shell of his own, along with an equally strong attack. The Kongōsōha are nowhere near as effective when used by Inuyasha due to the lack of the Wind Scar. During the following battle, he nearly defeats an onslaught of most of Naraku's enemies which included Kikyō, Kohaku, Kagome, Inuyasha, Miroku, Sango, Shippō and even Sesshōmaru. During this battle he broke Sesshōmaru's sword, Tōkijin in the process. He only flees after nearly losing one of his jewel shards.

One might speculate that at this time Mōryōmaru had become even more powerful than Naraku himself as he was able to put up a fair fight against so many of his enemies which Naraku wasn't able to do even with the greatest portion of the Shikon no Tama in his possession; however, Naraku is also cowardly and avoids fighting directly. This difference in strength was also noted by Naraku himself in his conversation with Byakuya. In the anime, Moryomaru had no trouble eating Meioju, who was actually alive in this version, and fought Sesshomaru one-on-one and broke the Tokijin at the cost of a crack in the armor. He gained the Diamond Spears when he caught a moth demon holding Shinta hostage; the Dragon-scaled Tessaiga was used on him personally instead of the vemonous demon.

Later, after Inuyasha gained the Senki, from a Nikosen, (for his Dragon-Scaled Tessaiga) Naraku sent Kanna to aid Mōryōmaru. He tried to absorb Ginka and Kinka, a pair of conjoined yōkai brothers (sharing an antagonistic relationship with each other not unlike that between Inuyasha and Sesshōmaru), as their blood would bond his armored shell together and prevent Inuyasha's sword from damaging it. Though he succeeds in devouring Ginka, Kinka allowed Inuyasha to absorb him with the Dragon-Scaled Tessaiga, thus giving him the "Flaming Tessaiga". The result is that when Inuyasha used Kinka's power against Mōryōmaru, Ginka's power struck back from within the shell. This greatly weakened Mōryōmaru, and he flees. Soon after, Miroku suggested that Mōryōmaru will discard Ginka's body (since it's become a weakness). In the anime, Mōryōmaru successfully absorbed Ginka and Kinka without any involvement with Kanna or Inuyasha's group, instead Bayakuya merely observes the fight and Sesshōmaru takes Inuyasha's place using the recently reforged Tenseiga.

Confrontation with Naraku
Sometime later, Mōryōmaru became involved in a plot by Naraku, and tried to steal Kōga's jewel shards for himself. However, he found himself at a disadvantage when Inuyasha and Kōga teamed up, with Inuyasha using the Dragon-Scaled Tessaiga's new technique to weaken Mōryōmaru, then Kōga using his Goraishi to break the surface of the armored shell. Even rearranging himself with the diamond spears wasn't enough to save him. Inuyasha broke through into Mōryōmaru's core and saw Akago. Before he could defeat the infant, Inuyasha was flung out when Mōryōmaru grabbed an extra jewel shard from Kai, a younger yōkai-wolf who was also caught up in Naraku's plan. With the shard in hand, Mōryōmaru escaped again, and reaffirmed his plan; to prove that he is superior to Naraku, just as Hakudōshi had intended to do until his death. A few days later, Mōryōmaru was seen to have regenerated, and escaped both Kōga and Kikyō, this time without fighting. Later in chapter 441 of the manga, Naraku ends up running right into Mōryōmaru, who finished recovering from his fight with Inuyasha and Kōga. The evil energy, emanating from these two god-like beings was so strong that when they met the forest below them and all life in it was annihilated by a huge amount of miasma even though they were only headed for each other. Both exchange banter with Naraku saying that Mōryōmaru can't kill him as long as his heart is within Mōryōmaru, while Mōryōmaru says that Naraku can't kill him either for the same reason. Chapter 442 continues with Mōryōmaru continuously destroying Naraku's body with the Kongōsōha that he absorbed from Inuyasha, while Naraku tries to absorb Mōryōmaru back into his flesh and reclaim his heart. Since Naraku can't break through Mōryōmaru's armored shell, he continues to cover Mōryōmaru's body. When it all seemed finished, Mōryōmaru's tentacles pierced the flesh and instead, Naraku's (surprised) face and his flesh are seen absorbed into Mōryōmaru instead. The chapter ends with Mōryōmaru, unharmed, saying "Naraku, I am the victor!” while the members of Inuyasha's group look on in shock.

Mōryōmaru then shifted his focus to taking Kōga and Kohaku's shards to complete the Shikon Jewel. His battle with Kōga and Inuyasha proved to be a stalemate as the Fuyōheki hid his demon vortex from view if his appendages were close to the Infant, and the Goraishi easily shattered his diamonds. Capturing Kōga, Mōryōmaru took off with the Infant furious that the Shikon Jewel wasn't granting Mōryōmaru more power. However, it was at that time that Naraku revealed himself to be alive and well, having allowed Mōryōmaru to swallow him to get passed his armor. When the Infant attempted to "abandon ship", Naraku took over Mōryōmaru and captured him. When Inuyasha cursed Mōryōmaru, Naraku destroyed the haku puppet after absorbing all his useful insides.

Physical Abilities

 * Possesses similar body-manipulation abilities as Naraku; he can form claws/blades/etc. from his body.
 * Attacks using spear-shaped tentacles (later on uses tentacles covered in Kongōsōha spears for protection)
 * Has a right arm that acts as a gigantic tentacle made of Kongōsōha spears, usually used to impale his opponents
 * He can protect his head from attack, by enlarging the two "hooks" in front of his armor.
 * Uses bat-like wings to fly, even though it is likely he can levitate without them.

Special Abilities

 * Live Body Pieces – Merging pieces of his flesh with other yōkai (and even humans such as Goryōmaru), thus giving him control over them and increasing their regenerative abilities. He often sends pieces of his flesh abroad to scout for new yōkai for him to add into his body.  His ability to manipulate discrete pieces of himself seems even more refined that that of Naraku himself; he is able to control numerous, widely separated pieces simultaneously, and with a high degree of independency.
 * Regeneration – Much like Hakudoshi and Naraku, Mōryōmaru can regenerate his body indefinitely, since he has no heart.  Whether or not the Infant would be affected by Mōryōmaru's body being destroyed is uncertain.  Mōryōmaru is never blown to pieces like the other two mentioned, but he is bisected by Koga, and vaporized from the neck down by Kikyo.  Neither instance harms the Infant.
 * Miasma – Mōryōmaru's miasma is used on numerous occasions, in much the same manner as Naraku or Hakudoshi.  It seems roughly as potent as the two aforementioned examples'.  When Mōryōmaru confronts Naraku, their miasma seems evenly matched, as a gigantic wave sprouts from their meeting point and immediately dissolves all local flora and fauna, but has no adverse effects on either of the two.
 * Absorption – Mōryōmaru wraps his flesh around a target and incorporates its power and abilities into himself.  He uses this ability to absorbe yōkai and make his body stronger.  Most significantly, he absorbs a yōkai named Meiōjū to gain a nearly impenetrable armor.  He also absorbs Ginka and Kinka, Inuyasha's Adamant Spears, and even Naraku himself.
 * Yōki Absorption – Absorbing his opponent’s yōki (demonic energy) and reflecting it back at them or simply consuming it to use as his own.
 * Meiōjū's Armor – After absorbing Meiōjū, Mōryōmaru gains an armored shell covering his entire body, save his face, and right arm below the shoulder.  He can manipulate the shell as he sees fit, as shown whenever Mōryōmaru sprouts tentacles from his back, absorbs a new demon, or attempts to release the Infant.  The shell armor is shown to be even more resilient than the adamant armor employed by Hosenki, and is capable of deflecting even the strongest of attacks.  The most damage it ever takes is a small crack, from Sesshomaru's Tokijin, which causes said sword to shatter from the stress.
 * Raimeihō (Thunderclap Cannon) – Fires purplish spheres of energy and tendrils of lightning; this ability is gained from the absorption of Meiōjū.  It seems roughly equal to the Kaze no Kizu in strength, and can counter Koga's Goraishi.
 * Kongōsōha Spears –  Mōryōmaru gains this ability from absorbing Inuyasha's Adamant Barrage.  He later uses it to great effect for the remainder of his life, most notably against Naraku.  This attack can penetrate all barriers, as not a single barrier in the series has been shown as able to withstand its force (excepting Tessaiga's when Inuyasha battles Sesshomaru).
 * Fire/Lightning + Kongōsōha Spears - After swallowing Kinka and Ginka in the anime, Mōryōmaru was challenged by Sesshōmaru. When attacking, Mōryōmaru coated his Kongōsōha spears with the twins' elemental powers.  However, their effectiveness is unknown, because Sesshōmaru promptly sucked them into a Meido Zangetsuha.  Mōryōmaru uses this attack again against Koga, which overpowers his Goraishi blast, but Koga dodges the fire-coated spears.
 * Shōki (miasma) + Kongōsōha Spears – After he absorbs Naraku, Mōryōmaru is able to upgrade his Kongōsōha attack by launching corrosive Kongōsōha spears infused with shoki. With the resulting increase in potency, he was once able to melt entire mountains in one strike.

Trivia

 * In his original Haku form, Mōryōmaru's wings were peach/tan, now, they are bluish-purple.
 * In Episode 6 (FA), it's implied Moryomaru has a small degree of free will, given the Infant told him to stop taunting Naraku and destroy him.

Mōryōmaru