When you get to the Underforge, Aela will go inside and once she's inside, you can go in. When you go in, you will see Aela in her werewolf form, and when you get ready, Aela's left arm is held over the fountain by Skjor, and you must drink the blood that came out of Aela, causing your form to change.

After the end of the meeting, you will wake up by the entry to the Underforge in werewolf form, causing all of Whiterun's residents to fear you. If you run into the roads, citizens will run into their homes and the guards will attack you. After a certain amount of seconds (Or if health is low), your form will change back to the original one.

You will wake up back in your true form (The Human Form) without anything equipped. You must re-equip your armor and weapon. Then talk to Aela the Huntress to enter the fort occupied by the werewolf hunters

Before you get in the fort's walls, use your beast form power and kill the werewolf hunters. You will have to use the unlock cheat on the cell doors to release the other werewolves. The first room is a sealed off area, so pull the chain. You will encounter more werewolf hunters. When you enter another hall, you will see a dead werewolf (Which is hanged on the wall) and you must proceed through the hallway and release the prisoners (If you forgot to transform) or sprint through the hallway (If you are in werewolf form).

When you get into the last room, you will encounter Krev The Skinner, the leader of Gallow's Rock. Aela must aid you (If you are in werewolf form) or you must help fight Krev (If you are in human form). Make sure you have potions!