I remembered what st_rev came up in a thread on nihilistic_kid as a brilliant crossover masterpiece.
Ladies and Gentlemen, may I present:
Harry tripped along cloyingly. He was on his way to meet his lover, Ron, for Valentine's Day. He smiled to see a Old one hopping along, carrying a blade in its mouth.
Harry was almost beside the slime-trail when he came across a Lovecraftean cake, lying alone on a cephalopoid plate. "That must be a treat from my cyclopean bear," he said to himself, and tripped over to it. The cake looked eldritch, so he ate it.
It gave him the most unnameable tingling sensation in his hand. "How unusual!" he said and continued tripping to see Ron.
When Ron came out to meet him, he took one look and fell over.
"What is it?" Harry cried trepidatiously.
"Your mouth! And your tentacle!" Ron said. "They're squamous! Can't you feel it?"
Harry felt his mouth and his tentacle. They were indeed quite squamous. "Oh, no!" Harry said. "I'm a woman!" He, or rather, she started to cry. "It must have been that Lovecraftean cake you left for me. Did you know what it would do?"
"I didn't leave you any cake," Ron said. "I got you a globule. It must have been that glutinous man who lives nearby. He acts a little mind-bendingly, ever since he gibbered a statue."
"But how can you ever love me, now that I'm a woman?" Harry sobbed.
"Well, I never knew how to tell you this," Ron said tremulously, "but I actually prefer women. And I think your mouth is really accursed like that."
"Really?" Harry dried her tears. Harry kissed Ron and it was an entirely unholy sensation, like the looming shadow that waits at the end of all our days.
They spent the night having entirely unholy sex, until the cake wore off suddenly.
Everything was rather awkward after that.
--you can manufacture one of your own here.