Herald
The Skill Gems commonly referred to as "Heralds" are a number of Active Gems which grant the user a damage buff and a special effect upon killing an enemy under certain conditions. Each Herald has a mana reservation cost of 25%.
List of Herald Gems
Possible future Heralds
During an interview in Episode 15 of the State of Exile podcast, GGG-Developers Carl and Rory mentioned that three more Heralds other than the then-released Herald of Ash and Ice were in the works. The names given were Herald of Thunder (Lightning), Herald of Stone (likely Physical) and Herald of Anarchy (likely Chaos).
Upon release of the next major patch after this interview, which introduced the Herald of Thunder as seen above, GGG stated that the other two Heralds were not ready and were put on an indefinite hold, as they were either not interesting enough or did not fit into the game for another reason.