That, if its hard, appears to be an infection that generally happens on the feet, and is called bumblefoot. There is treatment for it, but it's... bloody and icky and stressful... to treat it. You can take her to a vet, if you have one nearby that will take a chicken. Or you can tackle it yourself. The following is a description of how to treat it yourself:
You need to bring her inside and set up an area to be the "infirmary." Gather up gauze, something to wrap a wound with, like Coban, a sharp, sanitized razor blade, sanitizer to keep cleaning it with, paper towels or lint-free rags you don't mind getting stained, a washcloth or small rag to cover her head with, a medicine to put over a wound to help it heal (I use blukote, the liquid kind in a small bottle, not the spray kind), a tool like a pair of needle nose pliers or a hemostat to grab things and tug with, either liquid stitches, good quality butterfly strips, or standard stitches (I use the external ones, that you remove after a wound has healed), and a large towel you don't mind getting stained.
You then lay the hen on her side, with the injured leg up and pointing towards whatever hand you write with. Cover her head to help keep her quiet, and gently lay your non-writing arm over her body to help hold her still, but don't squash her.
Sanitize your hands, and the razor blade, and cut around large black spot on the bump, going into the regular flesh on either side. Now, there will be blood, but if it seems to be a heavy amount, use some of the Coban to go around her leg above the area very tightly, to be a tourniquet, to keep the bleeding under control. It will also help you see what's going in at the wound you're treating. Take the (also santitised) grabbing tool, and grab hold of that black bump and pull. There will be a big chunk of white, cheesy gunk come out. You have to keep hunting in the wound for more of that white cheesy gunk (that's the infection), until you are sure it's all gone.
You need to coat the area with the medicine at this point (triple antibiotics without pain relief in it will work, too, but I always use Blukote), and let it dry. Use whichever you havr chosen for closing the wound - liquid stitch, butterfly strips, or actual stitches, and stitch up the opening. Should only take a few stitches. Then get a few layers of gauze pad to cover the area, and wrap it snugly (not as tight as the tourniquet!) with the Coban. Let it sit for a couple of minutes, then slowly take the tourniquet off. Watch for bleeding through the gauze pads. If it starts seeping through, wrap another wrap or two of Coban a bit more tightly. Once you have the bleeding under control, check her toes. Find an area where you can see the color without it being buried under feathers. Pinch the area to squeeze the blood out if it, then release it to make sure there's enough blood flow to let it get its color back.
Then let her up. Watch again for bleeding. If all is well, put her in the infirmary and let her rest. Then go get yourself a drink... tea or coffee or an adult beverage of your choice. And congratulate yourself.
Go check on her regularly to make sure she's not bleeding. It's usually best to wait at least 24 hours to change the dressing, but not more than 48. Change it at that point every day, until there is no longer drainage.
As for antibiotics, there is very little that a person can purchase on their own that will work against this crap. I would suggest a vet to get antibiotics. Otherwise, my best suggestion is injectable penicillin for 3 to 5 days, at minimum, and if you can get it, switch to water soluble doxycycline for another 7 days.
This infection can come back! Keep a close eye on the area. Check her once a week, even... and hope that it hasn't spread elsewhere. It can get into the bone, and as high as this is, it could require the whole leg be amputated if it does get into the bone. That's where you would need to really make a decision. However, we will keep out fingers crossed that she recovers fully!
