Invoker

Invokers are a Divine Controller class introduced in Dungeons & Dragons 4th Edition, in the second Player's Handbook. This previously refered to Warlocks and Dragonfire Adepts in 3rd Edition. If you were looking for the Wizard specialized in blowing shit up, see Evoker. If you were looking for the DotA2 character, try /v/.

Fluff

Fluff-wise, invokers are beings who possess a direct and intimate connection to Divine Magic; this allows them to wield miraculous magics that other divine spellcasters cannot use. The possibilities as to why an individual is chosen are many, and invokers range from esoteric mage-priests to emissaries or agents of gods to the guardians (or avatars) of dead gods. To this end, they are capable of summoning angels, invoking divine symbols and speaking godly words of power, and raining fire, lightning, thunder and brilliance upon their foes.

"Regular" Divine casters like Clerics and Paladins don't really like Invokers, officially seeing them as using divine power not meant for mortals - unofficially, the fact that an Invoker's powers stem from direct investiture simultaneously gives them a greater presence amongst the lay faithful and means they are not beholden to the existing church body makes it easy for them to be worshiped as messiahs and prophets. It is very, very easy for an invoker to cause religious schisms or even internal wars if they don't like the way that shit works.

Add in that they got their powers from their birth rather than piety and contemplation, and the relationship between clerics and Invokers kind of mirrors the attitude Wizards have towards Sorcerers.

The closest analogy of this class in 3.5e is the Favored Soul, who is pretty much that edition's divine expy of the Sorcerer.

Crunch

Crunch-wise, they're Divine Controllers, with powers specialising in attacking multiple foes and casting down debilitation over a wide array of victims. Somewhat sturdier than the original Controller, the Wizard, Invoker powers focus heavily on Radiant damage, with a side-line in Psychic, Fire, Thunder and Lightning. They also dabble in the Leader role, with many Utility powers that bolster and augment their allies.

Like all Divine classes, invokers posses the Channel Divinity feature. They also possess the Ritual Casting feature, which means that, like a wizard, they start with a free Ritual Caster feat and a book with some rituals in it. Fitting their Cleric/Wizard hybrid concept, invokers use rods and staves to channel their powers, rather than holy symbols.

The subclass mechanic for invokers is the Divine Covenant class feature. This basically judges what kind of role the invoker's patron deity intended them for. Three Covenants were presented by the time 4e met its demise; Preservation and Wrath in the PHB2, and Malediction in Divine Power.

The Covenant of Preservation indicates an invoker charged with defending the faithful and seeking out allies. This Covenant plays up the invoker's Leader sub-aspect, granting an increased affinity for assisting allies.

The Covenant of Wrath indicates an invoker charged with destroying the enemies of the gods. This Covenant plays up the invoker's "Controller as Destroyer" angle, with an emphasis on inflicting greater damage with its prayers.

The Covenant of Malediction indicates an invoker charged with putting the fear of the gods into those who might oppose the divine order. This Covenant plays up the other aspect of the invoker's Controller role, with an increased ability to manipulate the positioning of enemies and a Channel Divinity that can inflict penalties on victims.

Uniquely amongst Divine classes, invokers do have an alignment restriction, in that they are supposed to at least begin play with the same alignment as their patron god. After all, they are direct agents for their deity, so they need to be spiritually aligned.

Invokers of Thu'um?

It's not quite as obvious as, say, the Runepriest and its love affair with runes, but the defining element of the Invoker is that they... well, "invoke". They speak divine words of creation, draw holy gylphs, and call upon the hidden names of gods and angels, with these mystic syllables turning words into power. It didn't go unnoticed by /tg/ that this is pretty similar to the concept of the Thu'um seen in Skyrim, which was released about the same time. Divine Power didn't help by introducing two Paragon Paths that further reminded /tg/ of the Thu'um-mastering order of the Greybeards; the Adept of Whispers, an invoker who attempts to master the power of speech (both divine and mortal) by only speaking when needed, and the Speaker of the Word, an invoker who has mastered a single divine Word of Power so potent that each syllable can kill and they never dare to speak the whole Word in a voice higher than a soft tone.

Was this intentional? Who can say? The Player's Handbook 2 came out before Skyrim did, but, given the long development cycle of Skyrim, and the long lore that both franchises have, it is impossible to say if someone ripped someone else off, or that this is just a coincidence.

Doesn't mean you can't play your Invoker as a Thu'um master if you want to, of course. Make sure to play a Dragonborn Invoker so you can be Dragonborn while you're Dragonborn.