Blazing Core
Paradoxically, it's only hot for those who does not touch it.
It can be found in chests generated within Nether Fortress.
While equipped in Spellstone slot, it will extinguish its bearer when they are set on fire, burn any creatures that attack them in melee, and provide them with immunity to fire and lava. Also, any status effects applied the bearer of the core will tick two times faster than normal.
The spellstone, in fact, burns any creatures that are considered dealing damage to the wearer directly. When this happens, the creature receives 4 points of fire-based damage, and is set on fire for additional 4 seconds. Obviously, creatures that possess fire immunity will not receive any damage.
The status effects ticking down twice as fast means that any effect that does something over time will also do it faster - for instance, Regeneration effect will restore player's health with doubled speed, and Posion will deal damage at doubled rate. However, status effect with constant traits will not benefit from this in any way - either Slowness or Strenght effects will simply run out quicker.