Mod-souls are pills that are the habitat of artificial souls made by the research of the 12th division. These souls were made to enhance human physiology making them capable to battle with hollows, with this they are able to double that of the power of an ordinary human and can even achieve zanpakuto but can not go any higher than Shikai level. Mod-souls must be bought and only one can bought per character, mod souls will have the same stats as you -1 point in every stat field unless human if human then the stats will double. In order for the mod soul to take effect it must be placed in a dead body which must be an PC dead body, a soul less object such as a plush doll or toy, unfortunately the dolls and toys alike must be bought separately. For shinigami they are able to make an artificial dead body for their Mod-Soul, however the mod-soul will never obtain a zanpakuto, which must be registered, until they have come in contact with an emotion equal to the soul they were artificially made to match.
Each mod-soul is a fragment of an emotion that was artificially made to match the creator of the Mod-Souls, ideally the creator of the Mod-Souls was the Ex 12th Division Captain as he went insane from extracting his emotions into each mod-soul. An other for a mod-soul to obtain the emotion to unlock Zanpakuto they must be in a near death situation with a hollow. Unfortunately this means they will be nearly dead when they gain their Zanpakuto, must train to unlock shikai later at 3,000 words and as rule of Shinigami states, they are not allowed to intervene.