Soul Fragment

The  is one of two types of material needed to craft the Soul Whip and Eternal Hoof respectively. It's dropped from killing the Grim Reaper, although it's not guaranteed on each kill.

This item is available in three-hour windows every Spooky Festival, more specifically when the Fear Mongerer is available in the hub. It's only recommended to fish for s at a Fishing level of 26 or higher, as that's the minimum level needed to fish up a Grim Reaper.

Trivia

 * The texture for this item is based off of obsidian blocks from Terraria.