For one reason or another you have made a pact with a lich for powerful magic. As part of terms of this pact you must harvest souls for lich in exchange for this magic. The lich has implanted a ritual in your mind that allows you to create a magical suit of armor that collects the souls of dead and funnels them into its phylactery. This phylactery armor is also designed to protect you and allow you manipulate the souls and their life energy to raise the dead and undead, heal yourself and allies, and unleash devastating spells from its magical built in eldritch cannon.
Many liches make this pact with more than just one warlock to increase the flow of souls they receive.
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