Out of all pregnancy that Liliah has, only the slug otherworlder have a successful birth as the slug is born with an eye. Meanwhile, the Father procreation with Liliah would result in stillbirth. What I've know that there is two ways to break the infertility blessing: 1. Failing to defeat the slime boss in dream and 2. from priest giving womb crest on Liliah. The priest of the cathedral possibly knew how to break infertility blessing due to possible previous association with Kessaria cathedral. Slime otherworlder may have put infertility blessing on Liliah to prevent competitors from taking Liliah as their host, and breaking the infertility blessing would induce deformities on competing otherworlders. Liliah may have took a contract with slime otherworlder in secret to boost her magical ability so that she can fight the Father in return for her to be able to give birth to a perfect vessel, which may explain why she embraced the slime at the post-credit ending when she decide to not kill it. Killing the slime at the post-credit ending may be seen as a betrayal for the slime otherworlder which may have a large consequence for her in the future.