What is the shirt of Nessus?

The shirt of Nessus is also known as the shirt that killed Hercules

The shirt of Nessus didn't kill Hercules or Heracles (his real name, Hercules is his Roman name) it was poisoned by Heracles' stepmother Hera (her revenge to Zeus for cheating on her) which she gave to Heracles' wife telling her the wearer will be eternally faithful and they would never be able to take it off (this part was not a lie). Once Heracles put it on though it tortured him/causing him great pain so he built himself a funeral pyre and lied down on it just then Zeus came and saved Heracles taking him to mount Olympus to become an Immortal/God