You merge the souls of a celestial/fiend/fey (your choice), with a willing creature with a C/R of 1/8 or lower. 
This will create a creature with two souls trapped inside, and depending on the circumstances could create a curious scenario. 

If successful, a second soul is summoned within the body of the willing creature, alongside the previously existing one. 
Refer to the rules in the Find Familiar spell.