Palace of Ghost is a weekly group instance that pits you against one strong boss, Cain. It can be cleared five times per week but only needs to be run once per week to unlock the raid feature to auto complete the other four runs.
This instance rewards materials for the new Equipment Memory system.
Once you reach base leve 160 the Place of Ghost is unlocked in the Instance finder.
Hellfire Embodied - If you die in any of the fire fields that Cain drops around the battlefield he gains a stack of Hellfire Embodied. Each stack reduces the healing received by the entire group by 20%. This can easily wipe the group so avoid dying in these fields!
Recommendations - Run fire resistance equips/cards to reduce damage taken. Use water elemental conversion or water based magic for maximum damage.
Defeating Cain rewards you with Memory Crystals and Sakray's Memories used in the new Equipment Memory system. He also drops a back piece to deposit for another 4 atk/matk, and on Epic Spirit difficulty can rarely drop a feather. Six of these feathers creates an extremely strong headwear like the one created from Angel Snow Feathers in Thanos Tower.
Unless you're part of a whale guild this is not something you'll jump into as soon as you hit 160. You will need a strong group to clear this unless you have an extremely strong account.