A Soul Weapon (Kor. 소울 웨폰) is a supernatural weapon that can be freely summoned by the Nobleclan leaders and the Lord. Soul weapons are the symbol of a Clan Leader's authority: every clan leader and the Lord possesses a soul weapon, and only they can wield them. However, there are several exceptions to these general rules. Rael Kertia is not a clan leader but owns an incomplete soul weapon. Cadis Etrama Di Raizel, the True Noblesse, is the sole high-ranking noble official who does not possess a soul weapon. The Noblesse is so powerful that his own "noble blood and soul", are his power and in their fully released state, are considered to be his soul weapon. Frankenstein is a human who owns and uses an artificial soul weapon. Regis K. Landegre was able to summon Legasus out of sheer willpower when his comrade was in trouble.
The origin of the term "soul weapon" comes from the fact that the previous wielder of the weapon leaves his or her soul and powers abiding within the weapon when they pass away, as part of the blood heir's inheritance. This way, they pass on their will and beliefs through these weapons. Soul weapons become more powerful with each passing generation, due to the accumulation of new souls and thus, the addition of more power.
A soul weapon amplifies the user's power and parameters drastically and usually has special abilities that suit their respective clans. However, it has been stated that soul weapons do not add power to the bearer of the weapon in the same way that a person wielding a sword gets additional power and abilities (such as additional reach and cutting ability). Rather, soul weapons are a very part of the wielder and embody the status which the wielder holds, as well as it being the main part of their blood inheritance. Wielding and mastering a soul weapon makes the wielder complete and whole. A noble clan leader or Lord that possesses an incomplete soul weapon is said to be incomplete themselves, akin to a werewolf lacking claws.
Soul Weapons seem to derive most of their powers from the wielder's aura, the ethereal energy radiating from the souls sealed within them. They can manifest this aura-based power as focused energy beams, force fields, making their wielder invisible, transmutation, etc. Soul Weapons also seem to be essential to a Clan Leader's "ultimate technique".
This technique is the ability to manipulate aura energy or one of its by-products or derivatives (e.g. the blood used in Blood Fields) in order to shape the souls sealed within the weapon into a physical manifestation of the clan symbol (e.g. the Grim Reaper, the Hydra, Cetus/Leviathan). After the clan's symbol has been manifested, it immediately attacks every enemy within sight with enormous power, enough to cause earthquakes and destroy multiple city blocks with ease. However, these manifestations can be dispersed with significant force.
In Chapter 180, a piece of information was revealed that gives a little insight into the nature of Soul Weapons. Frankenstein was genuinely surprised by the fact that Ragnarok has been preserved for over 500 years without an owner and concluded it to be related to the special properties of the hibernation temple. This implies that Soul Weapons cannot exist without an owner - or at the very least not for a long term - unless there are special circumstances to preserve them.
Also, in Chapter 311, Ignes deduces (she thinks she has it) that she received the Kravei Clan Soul Weapon immediately after the death of her father. This is consistent with the generally accepted theory that Soul Weapons are supernatural objects bound to the wielder's soul, and upon their death, their soul is absorbed into the weapon and becomes bound to the blood heir.
However, it should be noted that despite the fact that Roctis entered Forced Eternal Sleep, his soul was still sealed within his clan's weapon. This is also shown in the fact that the previous generation of Clan Leaders who all (with the exception of Gejutel and the six traitor Clan Leaders) entered Eternal Sleep (albeit not forced) and yet their souls became part of the weapon. However, if the previous wielder has died and does not have an heir, then it could be assumed that their soul will simply depart to the afterlife and the Soul Weapon is destroyed, releasing the generations of souls sealed within.
It has been hinted at that when not in use, Soul Weapons actually exist in an incorporeal state as an extension of the soul within another dimension. This is their 'default' state of existence and will continue to exist like this before being summoned into the physical realm in their tangible forms. Soul Weapons are summoned by the wielder expressing his will to use the weapon through a verbal summoning code or through the mere thought of using it. It also appears that Soul Weapons have a time limit to their use. After being summoned, they can be used for a certain period of time before having to return to their default state.
However, Soul Weapons can be preserved for extended periods in the physical world, most likely due to the fact that the previous wielder can infuse his/her aura into it for various purposes. This would imply that Soul Weapons need to feed off of the current wielders spiritual energy before finally absorbing their soul upon their death.
Soul Weapons also have a unique function that allows them to record and relay the messages, emotions, thoughts, and memories of the current wielder as well as the previous ones, whose souls are sealed within. This is most likely the main way that Soul Weapons act as a bridge between the past, present, and future, enabling its owners to pass on their will, experiences, and beliefs.
Soul Weapons can also pass on powers and abilities through another function. Whenever it is in use, Soul Weapons act as a medium through which the current wielder of the weapon can utilize a power or technique of their predecessors by coming in contact with and synchronizing their spiritual energy with their souls. This is possible because the soul is the abstract, conscious sum of all parts of one's existence, powers and abilities included. For example, Rajak was able to utilize his father's invisibility technique through the use of the Kertia Clan's Soul Weapon, Kartas.
Due to the fact that Soul Weapons are the vessels for vast numbers of souls, and are imbued with the spiritual energy of conscious beings (aura), they are candidates for being a new type of sentient beings. Proof of their sentience is the fact that they technically have a will and emotions of their own, created from the psyches of those whose souls are sealed within. These emotions and willpower can be actively felt by the current wielder of the weapon, the blood heir. This is evident in both true Soul Weapons and in their imitations. The best example of this is Dark Spear. Whenever it is summoned, the aura it radiates can be perceived by the mind as pure hatred, bloodlust, and wrath. Also, it's perpetually trying to absorb the body and soul of not just its enemies, but its user, for whom the souls trapped inside have a personal vendetta against.