The team is apparently discussing the rarity of Wrathful Invokers in Season 1.
Wrathful Invokers are the rarest of the four types of Malignant Hearts in Season 1. A player has brought up their rarity on Twitter and Global Community Lead Adam “Pezradar” Fletcher responded with the following:
Blizzard yesterday posted the official Patch 1.1.1 notes. The update is hitting live servers on August 8.