
Adamantine Dragon |

It depends on how you do the conversions.
If you convert them into unrecognizable variants with different names, then you can claim the work isn't a "copy" but is in fact a new work of art. However, if you end up with something recognizable and the original creator sues, then you put the question of "is it recognizably the same stuff" into the hands of a jury.
Posting the original text is a direct copyright violation, especially for the use you have here.
There is a "fair use" clause in the copyright law that says any published material can have (usually small) excerpts copied for legitimate purposes (such as doing a review). Posting a bunch of monster descriptions with the express purpose of putting your own versions of them out there would probably run afoul of even the "fair use" clause.
My recommendation would be "don't do this without the express, written permission of the original author."

Jeraa |

In general, you can't post copywritten materials. Thats why they were copyrighted in the first place - the original authors didn't want their stuff to be made freely available to everyone.
At least in the US, there are Fair Use laws that do allow a limited amount of copywriten material to be reproduced. However, what exactly is considered Fair Use is vague, and can only be determined by a judge. Even then, what one considered Fair Use may be too much for another judge.
Its just easier to not reproduce copywritten materials at all.