Special fruit from the Rotten Vale. One bite'll get you to the pearly gates.
How to get Heavenberry
This item can be obtained by finding the unique gathering point spawning only during "Upsurge: Crimson Fruits" in the Rotten Vale. Gathering it for the first time will unlock the Heavenberry ingredient at the Canteen(The quest The Lord of the Underworld Beckons has a guaranteed Upsurge: Crimson Fruits).
Just climb the vines on the left to find it next to some Herb: