Soul Piercing is a rare and mysterious phenomenon where a soul, or Shén (神, Shén), from another time, space, or even parallel dimension suddenly finds itself inhabiting a different body. Unlike Take Away (夺舍, Duóshè), where a cultivator's Shén forcibly replaces another's, Soul Piercing occurs due to a unique resonance or matching wavelength between two souls. Soul Piercing can be viewed as an astral migration, where the new Shén "pierces" into a body that is not its own, often driven by cosmic coincidences or unknown forces. The exact reason why Soul Piercing occurs is shrouded in mystery and can involve complex karmic ties, destiny, or even subconscious cries for help from one soul to another.
The key aspect of Soul Piercing is the perfect match between the spiritual wavelengths of the two souls. This match creates a bridge, allowing one Shén to cross into another's body seamlessly. Unlike Take Away, there is no violent clash or struggle; instead, the transfer happens smoothly, almost naturally. In some cases, Soul Piercing is believed to occur due to a subconscious agreement between the two souls, driven by a deep, often incomprehensible connection.
One soul might call for aid across dimensions or time, sensing impending doom or a pivotal moment in their fate. The other soul, resonating at the same frequency, answers this call, either willingly or unknowingly. This agreement happens at a level far deeper than conscious thought—it's as if their fates were always intertwined. Timing plays a crucial role in Soul Piercing. The initiating soul may be in a life-threatening situation, such as moments before death, unconsciousness, or a coma. Conversely, the responding soul might also be experiencing a vulnerable state, like near-death, coma, or even during their own moment of death. This synchronization of vulnerable states often acts as the catalyst for Soul Piercing.
Once Soul Piercing occurs, the invading Shén merges with the existing Shén. Because their wavelengths are the same, the body does not reject the foreign Shén. The new Shén absorbs the memories, skills, and knowledge of the original, allowing them to function almost as if they were always meant to be there. The new Shén often feels disoriented at first, struggling to reconcile their new reality with the echoes of the past life they've inherited. Due to the natural resonance, Soul Piercing is nearly impossible to detect. To outsiders, the affected cultivator might seem the same, albeit with subtle changes in personality or behavior.
Unlike Take Away, which leaves a noticeable disharmony between body and soul, Soul Piercing does not create immediate symptoms that can be easily identified. The new Shén usually becomes the dominant one, though this can vary. In some cases, the integration may be harmonious, with the new Shén willingly upholding the original's duties and relationships. In others, it may lead to a radical transformation, driven by the different experiences and goals of the invading Shén. Cultivators experiencing Soul Piercing might suddenly possess knowledge of advanced techniques, rare skills, or insights that would otherwise take decades or centuries to master, giving them an unexpected advantage—or burden.
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In here Soul Piercing is one of troupe that used in web novels, when a character suddenly woke up in a new body, especially at different world. Noted that this is different from 'Rebirth' troupe, it also have connection with 'transmigrated into', and 'quick wear' or 'quick transmigration' troupe. It was depend on the MC, whether they were born and raised in that body, or suddenly used that body and got memories from the original 'host'.
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I pluck this from my 1st handbook: The Xianxia Handbook: A Guide to Cultivation and Beyond, Volume 4 : Introduction & some, Chapter: Spirit/Soul/God (神, Shén) & Divine Sense, take away-etc.
For more full explanation about this, you can read that chapter.