Fusion is the ability to take on a monstrous form, be it a god, demon, or other mystical entity. An entity that is produced by the fusion ability is called a Fusion Monster. When one uses fusion powers their stats may change during battle. There are three ways that fusion powers have been used within the Shadow Hearts series.
A "harmonizer" (known as "harmonixer" in Shadow Hearts) is one who can fuse with the souls of creatures to gain attributes of that creature's element, as well as increased stats.
All four are blood related on his father's side, and all are harmonizers.
- Main article: Yuri Hyuga/Fusions
In Shadow Hearts Yuri can go into the graveyard inside his soul to strengthen his Fusion Monsters. The graveyard contains headstones that represent one of the six elements of the game.
To gain a new fusion, he must collect a number of souls corresponding to whichever element he wishes to power up, and touch one of the glowing, elemental headstones. In order to get the fusion, Yuri must first defeat it in combat.
The third tier fusions, are unlocked by finding elemental stones hidden around the world, and activating them by the gravestone. There are six in all - one for each element. They then must be fought, and defeated.
His non-elemental fusions are gained by two optional side-quests.
In Shadow Hearts: Covenant, Yuri again can go into the graveyard inside his soul to strengthen his Fusion Monsters. The majority of the rooms inside the graveyard contain altars that represent one of the six elements of the game.
When Yuri charges an altar for the first time he gains a new elemental Fusion Monster.
If he goes into the graveyard and charges the altar a second time he will gain a more powerful Fusion Monster of that element.
By using a Soul Drop on the altar after charging it twice Yuri will enter a battle with a spirit - when this is defeated he gains the third and final form for that element.
His non-elemental fusions are gained by different methods than described. For instance, the Fusion Monster Amon is gained by guiding Yuri through a plot-driven mini-dungeon that is accessed through the graveyard.
- Main article: Kurando Inugami/Demon Morphs
Also in Shadow Hearts: Covenant, Kurando uses fusion though it is called Demon Morph. Compared to Yuri's ability Kurando's is very limited, having only two possible transformations.
Saki Inugami is the only harmonixer encountered who is not a playable character.
Like her brother, Jinpachiro, she was born with the innate ability to fuse into beasts. During the course of Shadow Hearts: Covenant, she is possessed by the demon, Astaroth, who forces her to turn against Yuri and his party by transforming her into Kouenki.
Later, Saki transforms again into the beast to allow her son, Kurando to access to his secondary fusion.
In Shadow Hearts: From the New World, Shania can transform into a Fusion Monster by summoning a spirit she contracted with into her own body to unleash different powers. In order for Shania to contract a spirit she must battle it at a holy ground. After battling and winning against a spirit it will allow her to become a Fusion Monster.
After contracting a spirit a fetish affiliated with that spirit can be collected. There is more than one fetish per Fusion Monster whose effects and abilities are affected by the fetish they possess. The first fetish for a given Fusion Monster is received upon a new contract with a spirit, while the rest need to be found. Other fetishes include powering up special skills and attributes. Unlike Shadow Hearts and Shadow Hearts: Covenant, Shania has to locate her fetishes all over the United States, while Yuri has to power up his fusion spirits through the graveyard to receive a new and more powerful form or on the menu screen to receive skills.
Despite functioning the same as a fusion, gameplay-wise, Johnny's Awaker form is not a fusion, merely him being able to tap into his latent powers.
Soul Pacts (known as a Soul Contract in Shadow Hearts) are similar to Shania's Spirit Contracts, in that the entity that is summoned will offer their powers to the summoner. However, the difference between these and Spirit Contracts is that there is a great risk to pay for such power; namely, the summoner's soul and humanity.
It takes a strong willpower to avoid being possessed, or influenced by the pact being.
The summoned entity can be invoked at any time, but once defeated will fade away, often leaving the dying summoner in it's wake.
Many of the antagonists fought in the series have entered a pact with a demon. Known pacts include:
- Albert Simon - with Amon the God of Wrath.
- Dehuai - with a nameless god of Purgatory to turn into Yamaraja: Calamity.
- Wugui - with a revived god of war to turn into Yamaraja: War.
- Rasputin - with Asmodeus the God of Desire.
- Nicolai Conrad - with Astaroth the God of Destruction.
- Veronica Vera - with "The Queen of Darkness" to turn into Night Queen.
- Lenny Curtis - with the Godhand.
- Garan the Oracle - with Yama, King of Demons
- Raiden - with Raijin, the Japanese god of thunder and lightning.
- Hien - with Fuujin, the Japanese wind-god.
- Kato with the Japanese God-of-Storms, Susano-O.