735 ILCS 5/2-413
If in any action there are persons interested therein whose names are unknown, it shall be lawful to make them parties to the action by the name and description of unknown owners, or unknown heirs or legatees of any deceased person, who may have been interested in the subject matter of the action previous to his or her death; but an affidavit shall be filed by the party desiring to make those persons parties stating that their names are unknown. Process may then issue and publication may be had against those persons by the name and description so given, and judgments entered in respect to them shall be of the same effect as though they had been designated by their proper names. If there has been a person who may have been interested in the action, and upon diligent inquiry it cannot be ascertained whether the person is living or dead, it shall be lawful to make those persons who would be his or her heirs and legatees parties defendant as unknown owners, the same as if he or she were known to be dead, but in all those cases an affidavit shall be filed by the party desiring to make any unknown persons who would be the heirs or legatees of the person not known to be living or dead parties, stating that upon due and diligent inquiry it cannot be ascertained whether or not the person is living or dead and further stating that the names of the persons who would be his or her heirs or legatees are unknown. Process may then issue and publication may be had against all parties by the name and description of unknown owners, and judgments entered in respect to the unknown parties shall be of the same effect as though they had been designated by their proper names. Only one affidavit is necessary under the provisions of this section for the purpose of making persons described herein parties to the action.
735 ILCS 5/2-413