Spiritual Parasitism

A soul leeches off another soul

Disease

Soul Leech

Proximity to users of certain soul magics

Strength of will

Use of certain soul magics

Occurs when a weaker soul begins to grow off of or latches onto a healthy soul and drains energy

The host experiences intrusive thoughts more frequently and is more likely to act on them due to the parasite soul's influence

Development of OCD in some cases, other cases it can go unnoticed

Until the parasite soul takes over or fully drains the host soul, or until the host soul somehow recognizes the presence of the parasite soul and forces it out

Either the parasite soul kills or pushes the host soul out of the body; or the host soul forces the parasite out

Being careful with soul magic

The soul(s) need to be viewed by a soulseer to detect the parasite

Soul magic to separate the souls or pure willpower

Soulology

Very rare

Losing yourself in pursuit of faster recovery