“You who seek the Forge: your journey will be long, but your destination is closer than you think”. – Ouros
The Hallowfire Heart is pretty much the only Exotic designed exclusively for the elusive Sunbreakers, a band of fire-wielding mercenaries who lost faith in the city early in its existence. The Hallowfire Heart is one of the better chest pieces in the game and encourages you to utilise your Hammer of Sol frugally in order to collect massive benefits.
Hallowfire Heart gifts you with the perk Sunfire Furnace, which greatly improves the recharge rate of your Solar abilities while Hammer of Sol is charged. Basically, you will only activate this perk when your Super is charged and it will go away once your Super is used. Since it said greatly improves I decided to put that to the test and timed each ability both with and without Sunfire Furnace active. First of all, Class abilities are not affected either positively or negatively by it taking about 37 seconds to recharge regardless. Grenades and Melee are another story entirely because without Sunfire Furnace active they both recharge at around 1:15 mins but with Sunfire Furnace, your Guardian can use them again after 25 seconds, I would say that Hallowfire Heart greatly improves your ability charge rate but it comes at a cost.
There is no doubt that the Hallowfire Heart is a great Exotic, with an incredible recharge speed gifted to your abilities and the fact that Supers are generally reserved for the perfect moment makes it even greater. In PVE, the ability to rain grenades and melee attacks on your enemies is something that will allow you to carve out your territory no matter where you are, especially when you consider the additional burn damage. With a 25 second recharge time, you are almost getting your abilities back in a third of the time as long as your Super stays charged.
This benefit triples in the Crucible where abilities are one of the most powerful tools in any Guardians arsenal. The other is team shooting but that is another matter. If you can toss out grenades and melee strikes constantly you can be a deadly force in PVP. The problem comes with the fact that most Guardians only get their Super once per match and many want to use it pretty quickly or reactively to the battlefield around them, which nullifies Sunfire Furnace and puts you back into the same boat as everyone else, minus the Exotic slot.
Hallowfire Heart potentially can dominate in all spheres of Destiny 2 but it seems that PVE may be the best arena for it due to how often your Super can be topped up, which increases the benefits.
The Hallowfire Heart can be found from a random Exotic Engram decryption or from Xur if you are lucky enough for him to have it in his inventory.