You can use some powers on allies immediately and some you have to wait.
Specifically, you cannot use activated powers on an ally unless that ally has been in your party continuously since the start of your most recent turn. Activated powers are ones with the exhaust symbol (
) as part of their cost. All other powers can be used immediately.
One of the most valuable uses of Greatsword of Horrid Dreams is to prevent any powers that trigger off a card entering play. For example, Tatulla the Reclaimer's power won't trigger if your opponent has the Greatsword in play.
The hands of a healer