The Heart of Darkness is a Revengeance Mode-exclusive Pre-Hardmodeaccessory that fills up the Rage Meter by 30 per second and boosts the damage Rage Mode deals by 10% when activated. It also makes the player five times as likely to be granted the Absolute Ragebuff, and it increases the player's damage by 10% while they have it.
All boss bags have a 5% chance of containing any of the three universal Revengeance accessories, the other two being Stress Pills and Laudanum, and then a 1/3 roll is cast for which accessory is given. This results in a 1.67% chance to obtain any of the three accessories, however the player can never receive more than one at a time, and the drop rate is not affected by Defiled Rune.
This accessory is very useful for players whose play-style revolves around frequent use of the Rage Meter, as it reduces the amount of damage they need to take in order to enact their strategies.