is the original author(ushiki yoshitaka) even allowed to create something like this?

I know yumekui merry is his own work but...it's idk weird.

It's like if bleach/naruto/hunterxhunter/etc original author created 18+ stuff from his own series.

or is it a fake?

bruce1991 said:
is the original author(ushiki yoshitaka) even allowed to create something like this?

It's a photoshopped edit of a pic by the original artist. Check the parent post link at the top of the page.

It's been modified by a 3rd party, thus the photoshop and derivative_work tags.

While many artists create nude or otherwise explicit artwork of their own creations, they are rarely published. See Aaron Diaz's blog on the subject for an example of why nude figures are important in character design.