Look for fanfic communities dedicated to the source material that Phobos and Will are from. There, you can ask for recommendations of 'yandere phobos x will fanfic'. Members are usually quite helpful in sharing their favorite stories in this genre.
Well, 'That Time I Got Reincarnated as a Slime' can be considered in this regard. Rimuru, the main character, experiences an isekai adventure, and there are some characters with possessive or extreme loyalty traits that could be loosely related to yandere. Also, 'Log Horizon' is an isekai novel where the characters' relationships and their growth in the new world might have some yandere - like undertones at times.
There's also 'The Yandere Male Kagome's Quest'. This one is different as it focuses more on the male Kagome's journey while still maintaining his yandere traits. He goes on adventures, all the while being possessive and overprotective of the people he cares about. It's a unique take on the yandere concept in a fanfiction setting.
One popular 'yandere x kagome fanfic' is 'Obsessed Love'. It has an interesting plot where the yandere character shows extreme possessiveness towards Kagome in a rather unique Inuyasha - inspired setting.
Webtoon does have a few 18+ comics. However, their availability can vary depending on your region and the platform's content regulations. Some might be hidden behind age verification or specific access settings.
One interesting 'oc x yandere chan fanfiction' could be the one where the OC is a new student in the school and Yandere - chan becomes obsessed with protecting them from other students she deems as threats. It's a unique take on the relationship as it shows a different side of Yandere - chan's possessiveness.
No, webtoon and manhwa are not exactly the same. Webtoons can come from various countries and have diverse styles, while manhwa specifically refers to Korean comics.