The Heart of D'Sparil in Bright Crucible
The Heart of D'Sparil is a puzzle item available in Hexen level Bright Crucible. Since it is used in a secret level, the reference to D'Sparil, Korax's younger brother and final boss of Heretic, is considered an easter egg.
The Heart is found again in Constable's Gate in Hexen's expansion Deathkings of the Dark Citadel.
The heart appears as a red gem (in the image the heart is on top of the pedestal where it has to be put).