You may need to check your firewall. I've heard that they can sometimes go through and delete files they find harmful. But I have just heard this happening a few times so not really sure if this will help. ^^'
Well, first of all, do
not download any .dll files from websites that offer them in an attempt to replace the corrupt ones. They're often not approved by the official .dll file developer and come packaged with viruses and malware.
Secondly, one thing is probably causing all of your .dll files to become corrupt or missing. It could be caused by a virus, malware, another program mistakenly delete the file or purposely delete the files (such as a firewall as ArcticGalaxy mentioned above).
Here are some solutions you can try:
1. Scan your pc, make sure there's no malware or viruses running around on there that you haven't noticed.
2. Uninstall and re-install FH (seems like an obvious solution but may not work if your .dll files are being corrupted by an outside source)
3.
When you install software that uses the libois.dll dependency, the software should automatically register the file for you. In some cases your DLL file may not register properly, and as a result, will provide a "libois.dll not registered" error. Fortunately, you can use a built-in utility called "Microsoft Register Server" (regsvr32.exe) to re-register your libois.dll file.
How to re-register libois.dll from an elevated command prompt (Windows XP, Vista, 7, 8, and 10):
Click the Start button.
Type "command" in the search box... DO NOT hit ENTER yet!
While holding CTRL-Shift on your keyboard, hit ENTER.
You will be prompted with a permission dialog box.
Click Yes.
Type the following command: regsvr32 /u libois.dll.
Hit ENTER. This will UN-REGISTER your file.
Type the following command: regsvr32 /i libois.dll.
Hit ENTER. This will RE-REGISTER your file.
Close the command prompt window.
Re-start the program associated with libois.dll error.
Repeat this step for your irrklang.dll and ogremain.dll
4.
This may seem like an obvious or ridiculous step, but it is very possible that your libois.dll might have been accidentally deleted. Therefore, it is worth checking your Recycle Bin to see if it's there.
Simply double-click the Recycle Bin icon.
In the upper right-hand corner, search for libois.dll.
If libois.dll file appears in the search results, select it, and move it to the following directory:
Windows 95/98/Me = %SystemDiskRoot%\FeralHeart\
Windows NT/2000 = %SystemDiskRoot%\FeralHeart\
Windows XP, Vista, 7, 8, 10 = %SystemDiskRoot%\FeralHeart\
64-bit Windows = %SystemDiskRoot%\FeralHeart\
After moving your libois.dll file, restart your computer.
If none of these seem to work, lemme know and I'll look into it further.