In a modernised society, one would expect the need for spirituality to diminish; where once faith and belief were needed to answer questions, science has since taken that place. However, an understanding of the Ethereal Plane is still very much required: people still die, more than ever before, and the number of ghosts and other Ethereal denizens are increasing to match. As such, people will require someone with that particular expertise, and that's where you come in. Some that follow the Way of the Spiritseer may be spiritual leaders, while others may use their talents to become expert ghost hunters.

Ethereal Knowledge

At 3rd level, your focus and research on the Ethereal Plane culminates in an expanse of knowledge. You gain proficiency in either Arcana or Religion (your choice). You also have advantage on Intelligence checks to recall information about the Ethereal Plane, including about anything that lives there.

Wraith Strikes

Also at 3rd level, you can empower your strikes to overcome the natural protections of ethereal creatures. When you take the Attack action on your turn, you can expend 2 ki points to turn your unarmed strikes into Wraith strikes. Until the end of that turn, every unarmed strike that you make deals force damage instead of its normal damage type. In addition, when you hit a creature with a Wraith strike, you can expend 1 ki point to deal additional force damage equal to a roll of your Martial Arts die + your Wisdom modifier. You can only deal this additional damage once per turn.

Inured to the Ethereal

Starting at 6th level, your ki has tapped into the essence of the Ethereal, giving you a measure of protection against the effects of its inhabitants. You have advantage on saving throws against the spells and abilities used by undead. You are also immune to being possessed, and your hit point maximum cannot be reduced.

Spectral Sight

Also at 6th level, your ki impart your eyes with the ability to see as ghosts do. As an action, you can activate your Spectral Sight, allowing you to see 60 feet into the Ethereal Plane when you are on another plane and vice versa for an hour. Once you use this feature, you can’t do so again until you finish a short or long rest.

Swift Serenity

By 11th level, you have learned how to more quickly balance your thoughts and still your emotions. You can now use your Stillness of Mind feature as a bonus action.

Compelled Serenity

Also at 11th level, your study on the way ghosts overpower the spirits of other creatures has granted you new insights. You can use your Stillness of Mind feature as an action on a creature within 5 feet of you. When you do so, you can choose to end one effect that is causing that creature to be possessed instead of charmed or frightened.

Ethereal Inhabitant

When you reach 17th level, your physical form begins to take on traits of the Ethereal Plane. You become resistant to cold and necrotic damage, you gain a flying speed equal to your walking speed and can hover, and you can use an action to enter the Ethereal Plane from the plane you are currently on, or vice versa. You are visible on the plane you entered from while in the Border Ethereal, but you can’t affect or be affected by anything on the original plane.