Find the Tower and loot the chest.
The Tower can be easily found due to the Crest of the Hollow Knights atop of it, along with 2 Moonstone Blocks and its unusual design. You will face Hollow Knights of Fire and Hollow Knights of Thunder. The chest will contain 2 Energy Crystals and other drops based on the event's level. You may also grab 2 Crest of the Hollow Knights on the ground floor, one at the the second floor and the last one near the chest.
- In case you don't want to face any of the creatures you can rush to the chest, loot and reenter the island. All the structures will still be in place but not the foes.