Grace is an ability able to be used by a select few memebers of the Secret Tribes, along with Guardians and Nightmares. Grace was actually created by Soulfllies, but it spread because of their interactions with other creatures.
Grace allows the user to be protected from Special based attacks. An example of this being a Fire element based attack, on normal conditions it would destroy the receiver, burning them completely. WIth Grace however, the receiver still feels the heat and gets a few burns, but overall protects them from total destruction.
Grace also can protect smaller creatures when devoured alive. It gives them a chance to escape unharmed. This helps the Soulflies, due to their small size, but it also can consist people who battle large creatures on a daily basis.
While Grace is very powerful, it can only be used for a few hours, and takes another twenty-four hours to recharge. Soulflies are the only know creature to avoid this limit. Because of this, it also gives the alternative term Grace Period.
However Grace is said to appear sometime in the GALAXY series.