My understanding was always that this should be an action for a regular turn.
While the RAW doesn't specify, I've always interpreted it as being done immediately. That way if it's the PC using it that takes the severe consequence that triggers it, there's no opportunity for him to be taken out before it happens. Plus it feels more dramatic that way.
Does it take up said character's normal action then?
This has never come up for us, because the character using it was always taken out before his next turn. (Which is why I don't make him wait, see?) But, I would still allow him a normal action if he ever survived till his next turn. My reasoning is, again, because it's a little more dramatic that way, also because it takes little to no time to mutter a prayer and the character isn't doing anything after, his god is. I just don't see any reason to make it cost him his turn. If it were being abused in my games, I'd feel differently and my house rules would change.