The Invoker has no precedent and no update (so far). It first appeared in Player's Handbook 2 for the 4th edition and it is described as a Divine controller than channels raw divine power. In a way this makes them the divine counterpart to the Wizard; an arcane controller.
It could be described as the "Fist of the Gods".
What makes the Invoker an interesting choice is not just it's role, but the fluff text that goes along with it. So an Invoker would worship or honor a god as part of the whole pantheon. The examples given state an Invoker of Bahamut would also likely honor Tiamat. Invokers are all shown worshipping older gods; not Old Gods or even Old Ones...but the first gods that are still worshipped today.
In fact most invokers would be the tireless enemy of any "Old Ones". In the 4e fluff they are described as the human/mortal agents of the Godswar vs the Primordials. Replace that with Titans, or Old Ones or even demons and you get the idea.
What strikes me most about this class is how well it could be added to any old-school or 5e game.
In 1st/2nd Ed it would be a sub-class of the cleric. The invoker can turn/rebuke undead like a cleric and it also has access to divine spells. Of course you could make an invoker easy enough with good role-playing and a conscious choice to take damage dealing spells and limit yourself on the healing magic. Since invokers are much more of a "kill them all and let the gods sort them out" type of holy warrior I would also say that Raise Dead, Resurrection and Reincarnate are out of the question.
For their spell lists. Well the powers/spells in the 4e PHB2 are very colorful; Blades of Astral Fire, Glyph of Imprisonment, Cascade of Five Suns. Most do damage based on Wisdom modifiers (which in 4e includes level). If I were to create an old-school Invoker I would give them some invocation spells from the wizard's list. Not more than 1 or 2 per level though. I would make sure they had plenty of damage causing spells they can do from a distance. Plus the spells all need to have very grandiose names, such as "Righteous Fury of 10000 Blazing Suns" or "Wrathful Vengeance of the Plough God" (Plough god? Yeah, think about how important agriculture, growing and food was/is to people). These are not hippy clerics of the "God is Love" type, these are the Old Testament, destroying people in a genocidal flood types.
The closest thing Pathfinder has to this concept is the Inquisitor. While their missions are the same, the invoker is believed to have a piece of divine power instilled in them while the inquisitor is all too mortal. Still though there are some good ideas for some more spells on the inquisitor spell lists.
I think the reason I like the invoker so much is that it really is how I always liked to play to some clerics. I have played several clerics over the years. First first character was a cleric of the "investigate and destroy evil" type. I have played a number of paladins as well. In fact my 3.x edition one was known as "The Fist of Pelor".
This class appeals to me as another sort of cleric. One that is more action and dealing damage than one that is contemplative and healing damage.
Plus I forgot how much fun some of the 4e stuff really is. There is a lot going on in this game and it is a shame it will be tossed aside as a failed experiment.
BTW +Jonathan Becker has been running a series of posts on his thoughts of 4e. If you have played the game it is interesting to see it through the eyes of someone experiencing it for the first time. If you have never played it then it is worth your time. He has not gotten around to the invoker yet, but I am curious to see what he has to say.
