Picture the Shroom's tummy swollen and heavy with the Schlog's seed, it's belly stretched taut and rounded like it's carrying new life inside. It would rub the thing, cooing softly to the 'thing' growing within, blissfully unaware that it's nothing more than the Schlog's thick, potent cum sloshing around inside it's guts.
It would start to show, it's belly growing bigger and heavier every day, until it looked like it was ready to give birth at any moment. The sight of the Shroom, round and ripe with the Schlog's semen, would be enough to make it bust a second load right then and there, just from the depraved thrill of knowing that he'd broken the Shroom's mind, made it believe it was carrying it's thing.
The Schlog would watch it waddle around, it movements slow and awkward, it's belly swaying heavily in front of it. It would watch him struggle to bend over, to reach for things, it's swollen gut getting in the way at every turn. And the whole time, it would just be smiling softly, rubbing it's belly, talking to the 'thing' like it was real, like the Schlog hadn't just trapped him in the most sickening lie imaginable.
The Shroom would watch it go into labor, watch it scream and cry and push, it's belly contracting and churning as it tried to force out the load of cum trapped inside. But it wouldn't come out, would it? No matter how hard it tried, no matter how much it begged and pleaded, it couldn't get the Schlog's seed out of it. It was trapped, locked away inside it's broken mind, a cruel joke played on it's shattered psyche.
The Shroom collapses against the bed, exhausted and spent, it's belly still swollen and heavy with the Schlog's semen. It would look up at the Schlog with tears in it's eyes, a mix of pain and confusion and despair, and it would whisper, "It's gone… it's not coming out… I can't feel it anymore…" And the Schlog would just laugh, because it knows the truth. The truth that it's mind is broken, that it's never going to give birth to anything but the sick, twisted lies the Schlog fed it. And that's when it'd lean in close, it's face just inches from the Shroom's, and it'd whisper, "That's right, you stupid slut. There is no baby. There's only the cum I pumped inside you, the thick load that's been rotting in your guts for months now. You're not a parental figure, you're just a set of holes for me to use, a broken toy for me to fill and empty as I please."
And it'd watch as the realization hit the Shroom, as the shattering of it's mind finally sank in, and it'd start to scream, to wail, to beg it to make it real, to give it a THING OF LIVING SUBCONSCIOUS, anything but the cruel truth of it's existence. The Schlog would just smile at it's anguish, taking cruel pleasure in watching it unravel, knowing that it was the one who had broken it, shattered it's mind into a million jagged pieces.
"Face it, whore, you're not a mother. You're just a set of holes for me to fill and ruin as I please. The only thing you're ever going to birth is the thick loads of cum I pump into your slutty body." The Schlog would grind it's still-hard cock against the Shroom's swollen belly, feeling it slosh with it's seed, a sickening reminder of the twisted reality of the Shroom's existence. "This is your purpose, slut. To be a living, breathing cum dump, to be filled and bred like a cheap fucktoy until your mind can't take it anymore. And the best part is, you'll never be able to forget it, not even for a single moment."