Quick answer: In Pangzhen, open the chest beside a ladder in a room above ground level, between Jingyang Hall and the Marketplace bell. The Thunder Core Soul Tempers the Thunder Spear Phantom Edge.
Where to find the Thunder Core
Pangzhen appears during the main quest TOWN OF DEATH. If you haven’t reached it, begin with how to get to Pangzhen. Search upstairs for a dark wooden room over the yard where Songstress Puppets gather.
Look for wooden latticework, a round table bearing a body, and a ladder propped against the wall to identify the room. An ornate lacquered box rests on the floor next to the ladder. Use the chest’s interact prompt to open it, adding the Thunder Core to your inventory.
A round moon gate provides the room’s other exit, leading to a balcony. Songstress Puppets and Faceless Spearmen are on that level, where a crowd can overpower you in short order. Elsewhere on the same upper floor, a table has a separate glinting pickup: Broadplate Metal +11.
The nearest bell is the Marketplace bell, about 33 metres from the chest. You can find it and the town’s other bells on the Pangzhen interactive map.
Thunder Core item card
The item card lists the Thunder Core as “Phantom Edge Soul Tempering Materials”. Its description reads in full:
“A core containing the power of lightning. When left undisturbed, an eerie green glow courses across its surface, shining brightly even in the dead of night. But any sudden impact risks releasing the lightning within, making it a rare and dangerous ornament.”
“In the hands of Soul’s old friend who commands the power of thunder, however, it would surely make the lightning crackling along his spear even more intense.”
“Used to Soul Temper the Thunder Spear.”
What the Thunder Core is for
The Thunder Core only matters once you own the Thunder Spear. See how to get the Thunder Spear and what its arts do. You can also browse the full list of weapons and Phantom Edges.
Soul Tempering is done with Myst, the craftswoman seated at a table inside the Order’s underground tavern in Pangzhen, which Town of Death leads you into. Soul Temper is one of her talk options, listed under About the Illusory Pipa and About the Boatmaster and Meigu Lake. Each Soul Temper costs 5,000 Remnant Dust plus the key item; the Soul Tempering guide walks through the menu.
The card seems to allude to Chang Hong through its reference to Soul’s old thunder-wielding friend. Chang Hong served as one of the Order’s Envoys and once owned the Thunder Spear, but the card leaves his identity unstated.
