What if I had a party and someone got hurt?

Both you and the landlord could be sued depending on the facts. Was the victim drunk? If he or she was drunk they may be found to be comparatively negligent in causing their fall. Was there a defect in the stairs that caused the fall? Was there inadequate lighting? There are too many facts about the fall that have not been stated to offer an opinion on the merits of a case brought by the accident victim. Was this an apartment? If there was a dangerous condition of the premises that were under the control of the landlord he could be sued. If you had any duty to control the stairs, you could also be sued.