Spiritualism is a new talent added in the Great Reckoning Part 2 (version 1.61 LIVE), as part of the talent rework. With it, you can perceive invisible spirits in the world, and either exorcise them for Roleplaying EXP, or allow them to possess you for various bonuses.
Your Perception subpath determines the distance at which you can perceive spirits. When you see one, it will be 'paged', showing it as either white (good) or black (malevolent).
Governed by the Release subpath, upon finding a spirit, you can interact with it and choose to send it on its way to Lazarus, which requires some Mental Stamina. Depending on your Release subrank, you may succeed or fail, which may have negative consequences based on the spirit's page (black ones may retaliate for example).
Upon success, you receive Roleplaying EXP equal to 1% of your required TNL.
An alternative, although foolish, option is to let the spirit possess you. While a spirit is possessing you, you are treated as the Possessed racial type, although you retain your racial skills, as well as control over your character. You can maintain possession of up to X spirits (equal to your Possession subrank).
Once possessed by a spirit, it will give different beneficial effects based on its page. For example, White spirits grant HP and FP, as well as a rank bonus to one active skill, while malevolent Black spirits will grant you damage bonuses against certain races.
If you are possessed by spirits, when you are reduced to 0 HP, some of the spirits may escape your possession and go on their way. The only exception to this is if the Spar option is chosen in PvP.
The effects of spirits possessing you, as well as options to release specific ones or all of them are available in the occult box on the character panel. Simply click the Spiritualism icon.