Fun fact: (I think) Johan Nohr came up with that. For fun, I tagged the Mörk Borg Twitter account with the picture of the half-troll and asked how they are made. I believe "when a half-dead troll with a belly full of half-dead humans gets a chance to heal" was pretty much the verbatim answer :D So in my mind, when a troll swallows a human alive and only has a few HP left, they fuse to become a half-troll.
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Oh yeah, I found the Tweet:
https://twitter.com/MorkBorg/status/1484197505218879493
"Half-trolls are made when a half-dead troll with a belly full of half-dead human get a chance to heal. It’s not pretty."
Interesting!