Malignant Invoker

The odds are good that if you're playing Diablo 4's Season 1 content, after you skip or complete the campaign you'll find yourself diving into the Malignant Tunnels in search of Malignant Hearts -- while none of them will make or break a build quite like legendary aspects, they provide varied and powerful effects.

If you're playing Diablo 4 Season 1, the Season of the Malignant, and you haven't been able to get Malignant Hearts to drop for you, that may be because of the way the Season theme works this time.