Find an Ancient City in the Deep Dark biome
Locate the Warden’s structure area inside the Ancient City
Search the city’s chests for a Heavy Core
Open every chest you find, since the Heavy Core is only obtainable from Ancient City chests
Use Silk Touch or other tools only if needed for safe exploration, not for the Heavy Core itself
If you do not find one, loot more Ancient City chests in other Ancient Cities
Use the Heavy Core to craft a Mace with a Breeze Rod and a Heavy Core