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I think it's a pill. It's some kind of herbal thing that only the Beom family can use to give their partner the ability to have children. And it magically makes a uterus (if I remember right, I haven't read it in a minute). I can tell you two things with absolute certainty:
1. Go Yumin DID NOT know beforehand that Beom Taejun was going to give him the herbal thing.
2. Beom Taejun did this without consent because he wanted to make sure Yumin couldn't leave him. Regardless of Yumin wanting to have children with Taejun at some point, Taejun absolutely did it with the goal of baby trapping Yumin.

I READ CHAPSTERS 22 AND 23 AND TAEJUN CAME INSIDE HIM???? IS HE GONNA BE PREGNANT SOON WHAT.