First, Build the Wrath of the Ancients bow. Click this link on how to do that.
Second, go on near double tap then look up on the ceiling where you will see a mark. Shot using your bow and the first part will drop.
Third, once you have the part, go below the tower (Trophy Room) and you will see a glowing purple on the floor. To break that, you need to kill zombie by melee will the zombie stand on that tile. If you did this correctly, floor will break and once you press action button. The base or lantern will come out floating on the mid air.
Fourth, Six Skulls. There are six locations that you need to pick up and this can be done in any order. First location, near mule kick right next to the broken walls (where you will see a zombie died on the table next to the wall). Second location, next to double tap, and its on the wall (its on the corner). Third location, samantha’s room inside toy chest. Fourth location, second level of the upper courtyard to your right before you reach the door and its on the window. Fifth, teleporter room and its on your right sitting on the sink. Six, launch pad area. Its in the back of the truck.
Fifth, on this step you need to make six crawlers and bring them on the trophy room. Once those six are inside, skulls will kill them and. Voice will say Door, Horn, etc… Next, we need to get three symbols. This symbols will drop randomly will killing a zombie. Once you complete this symbol, head over next to the control room and you will see a knights (between control room and power room). Below this six knights there are a image that represent the quote that has given while ago. Press action button to reveal the right code that you need it later. Head over back to the Trophy Room and shot those symbols on the floor using your bow and if you did this correctly, you can now get your Reforged Arrow by press action button on the purple mist. Note: you can check back again the last give quote by shooting the lantern. This Codes are randomly change each game.
Last, bring this Reforged Arrow on the basement and place it on the box that represent your arrow. Kill 10 to 25 zombies, once its done. Place your bow on the box and now you have upgraded skull or purple bow (whatever you want to call it).