I Tried this one out as soon as i saw this and your going to have to make a custom ability from metamorphasis. This Abiliy changes the Demon Hunter into a Demon So Playing Around With it you can make grom hellscream into his chaos form all you need to do is:
Select Metamophasis in the object editor in the Abilitys tab.
Then Click the custom ability button and it will automaticly put up a window with all the abilitys on it (and if you Clicked the Metemorphisis) then it should so up with metamorphisis already clicked.
Scroll down until you get to the Data Files and you will find the Tab : Data - Alternate Form Unit Change This to Grom Hellscream (Possesed). Further Down You Will See Data - Normal Form Unit Change this one to the Actual Grom Hellscream.
(NOTE: This Ability Can Only Be Given To Grom Because It Will Transform BACK Into Grom!!!)
And you can change the Icons and Tool Text Any ay You Like!!
Rise of the Demon Kings! Coming July 2009!!