Unless stated otherwise, everything here is licensed under Creative Commons Attribution 4.0 (CC-BY-4.0). See CC-BY-4.0 License for more information.
This means you are free to use the artwork in any way you want. The only thing you have to do is, giving proper credit to me in your software's credits screen (in software) like this:
Includes Guardian Monsters Artwork by Georg Eckert / limbusdev
There are very few exceptions, but they must be named here.
The Guardian Monsters Logo is copyrighted. All rights reserved. Please do not use it.
You are allowed to use the monsters in any way you want. But you must use the given names, to keep them consistent everywhere.
If you have any problems with the license, or if you are not sure, if you're allowed to use them for your project, simply open an issue and ask ;)