Devil's Rubber Underwear, Rubbery Demon TF, 2Page (Patreon)

2020/05 works.

It's like... you put on these mysterious pants that are supposed to make your body stronger, but then you feel a throbbing pulse and it's as if you're being caressed. Before you know it, tentacles start growing from the inside, squirming their way into your body, fusing with and eroding your cells, turning you into some kind of rubber demon (?)!!

I thought it might be nice to draw a standard human-to-human-shaped demon transformation for once, but as I started sketching, things began to writhe inside, and tentacles and muzzles burst through the skin. That's life!

It's super unclear, but the 5th panel (left side of page 2) is meant to show the final stage of the process, where the underwear gets sucked into the privates, completing the fusion.