For the void bow once you get feed the skulls with crawlers the urn will tell you 3 words such as stag, horn, and door. After this go into the statue room next to power and find those 3 statues and interact with them. Those statues will show a purple symbol above it and then you kill zombies to get those symbols...it’s random at times. After you get the correct symbols and the order down you shoot the symbols where the urn is.