I was asked to attend a discipline hearing immediately after I returned to work for filing a violence in the workplace on my supervisor.

The timing of the discipline hearing suggests retaliation for your complaint.  Nevertheless, you would still need to attend the hearing.  Listen carefully to what they are claiming you said or did, ask for any documentation they claim to have to support whatever their claim is against you.  Unless it is something that is totally crazy (in which case you can easily deny it), you should ask time to prepare a written response to their allegation explaining that you feel strongly that this is retaliation for your complaint and so you want to make sure there is no misunderstanding (or re-wording) of your position.  You should not sign anything except to note that you have received a copy of whatever document they want you to sign.  Your signature should note you are signing only to acknowledge receipt and does not indicate you are agreeing to anything in the document. You should meet with an attorney to discuss this further to determine what legal action  you can take