Here's a vintage card from the Museum of Witchcraft at Boscastle in Cornwall. I picked it up in the early 90s when the museum had a Hammer House of Horror type ambience.
"To give is to receive". A young witch offers the Horned God the foetus, proof of her conception with the Spirit Force from the World Beyond the Grave. In return the Horned God will assign to the witch a familiar Spirit or Guardian Angle to serve her and to be the catalyst of her magical power.