What is Invoke in Hearthstone?
Hearthstone has many card effects that are common enough that they get their own keyword - a word or two that immediately tells you how the card works. Let's take a look at the Invoke keyword.
Use Galakrond's Power.
Invoke is a keyword introduced in Descent of Dragons as a centric piece of Galakrond. When you play an Invoke card, you will advance your Galakrond hero card (for every 2 Invokes, Galakrond upgrades one form), and it have the same effect as your Galakrond's Hero Power (for example, adding a Lackey to your hand if you're playing as a Rogue).
Invoking is fairly reminiscent of C'Thun and his synergy cards from Whispers of the Old Gods.
Keywords that Work Great with Invoke
As Invoke is a mechanic that simply only relates to a Hero card, there's no keyword that specifically synergizes with it. All Invoke-related minions and Galakrond Hero cards have Battlecries though, so any Battlecry synergy could be useful.
Playing Against Invoke in Hearthstone
When your opponent Invokes, you will see their Galakrond status on the left of the screen. Use this to take note of what stage your Galakrond is at. If your opponent plays Kronx Dragonhoof while they aren't Galakrond, you will now know they have their Galakrond in their hand. Since they are most likely planning on playing it next turn, a Boompistol Bully can delay it for one turn and buy you some time. This also works the opposite way, in which they won't be able to play Kronx while they are Galakrond and can't unleash a Devastation.
Example Invoke Hearthstone Cards
Shield of Galakrond is arguably one of the best Invokers in the game. It doesn't lose too much stats for the ability, and the ability is really useful. An auto-include for all Galakrond decks.
Corrupt Elementalist is an interesting one. It's the only one that actually Invokes Galakrond more than once (thus guaranteeing an upgrade if it isn't already fully upgraded). It, along with the entirety of Galakrond Shaman has seen less play since nerfs.
Seal Fate is an Invoke effect placed on a spell that deals 3 damage to an undamaged character. Otherwise though, it isn't any different from other Invoke cards.
Fate Weaver doesn't Invoke Galakrond himself, but instead provides a special effect if you've upgraded your Galakrond at least once this game (as it requires 2 Invokes to do that). Its Cost reduction effect provides Galakrond Priest its combo potential.