735 ILCS 5/18-103
The People of the State of Illinois shall be deemed the plaintiff and the adverse parties shall be defendants, and the first pleading by the plaintiff shall be designated a complaint. The complaint need not set forth the basis of the challenge, but may in general terms allege that the defendant is exercising the claimed right without lawful authority and call upon the defendant to show by what warrant he, she or it exercises it, and if more than one ground exists they may all be joined in one count.
When the complaint is filed by a citizen on his or her own relation, it shall be alleged therein that his or her requests of the Attorney General and the State's Attorney, respectively, to bring the action, have been refused, or that they have failed to act, as the case may be, and that leave of court to file the complaint has been granted as provided in Article XVIII of this Act.
The several rights of diverse parties to the same office or franchise, privilege, exemption or license, may properly be determined in one action, and all such persons may be joined in the same complaint, in order to try their respective rights to such office, franchise, privilege, exemption or license; but the court, in its discretion, may order separate trials when convenience in the determination of any of such rights so requires. No matters not germane to the distinctive purpose of the proceeding shall be introduced by joinder, counterclaim or otherwise.
If the plaintiff elects to set forth expressly in the complaint the grounds for an attack on the defendant's claimed right, the defendant may answer the complaint or present a motion directed thereto as in other civil actions, but if the complaint is in general terms, as provided in Article XVIII of this Act, the defendant shall by answer disclaim or justify, and, if the defendant justifies, shall set out the facts which show the lawful authority to exercise the right claimed. The plaintiff may reply to the answer or present a motion directed thereto as in other civil cases.
735 ILCS 5/18-103