Mod of Redemption/Demonhollow Core

The Demonhollow Core is a Pre-Hardmode. When equipped, it grants 5% increased magic damage, immunity to lava and most types of heat, boosts to all stats while in the Underworld and being submerged in lava greatly increases life regen. However, your life regen is nullified on surface and you will lose life at a quick rate while submerged in water.

Trivia

 * This accessory, as it suggests, is an accessory essential for underworld explorations and battles within there. So, equip it whenever you go underworld or in underground to get the most beneficial stats.
 * As it also boosts magic damage specifically as well, this accessory can be really useful against Wall of Flesh if you are going mage.