FUD is acronym for fully undetectable.It is a software that can be used to encrypt your exe files.
What is the use of FUD Crypter?
FUD crypters can be used to encrypt viruses,RAT,keyloggers,spywares etc to make them undetectable from antiviruses.
The Basic Working Of FUD Crypter is explained below
The Crypter takes the original binary file of you exe and applies many encryption on it and stores on the end of file(EOF).So a new crypted executable file is created.
Original Exe Crypted Exe
001————- 010                                      101————-110
100|Original File|000->  Cryptor  ->010|Original File|110
010————- 111                                        110————-010
The new exe is not detected by antiviruses because its code is scrambled by the crypter.When executed the new .exe file decrypts the binary file into small the data small pieces at a time and injects them into another already existing process or a new empty one, OR it drops the code into multiple chunks in alternative data streams(not scanned by most a/v) then executes it as a .txt or .mp3 file.

Where can I Download a Free FUD crypter?
 
as the crypter becomes popular it doesnot remain FUD.So the only FUD crypter available are those made by indivuals and they can be found by spending a little time on google by searching