Malignant Tunnels

Diablo 4's Season 1 added a new repeatable boss fight, Varshan, the Consumed, which first shows up during the new Seasonal storyline -- but after you defeat him and finish the questline, you'll learn how to begin crafting items that allow you to repeat the process, and there are some great rewards for doing so.

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.