You’ll need to track down multiple resources while playing Hades 2 as they’re needed to unlock other weapons or you’ll need to use them for special spells. Cinder is one of the harder ones to find.
Hades II – Time Cannot Be Stopped
Cinder is similar to other sources in rewarding your efforts in defeating enemies. But it’s unique in that you can only unlock it after defeating a specific encounter in Hades 2. You can only earn one per run, which works a little differently than you might expect if you played the first Hades.
You can only unlock Cinder by defeating Hecate in the Erebus region of Hades 2. This is the first area you reach before moving on to the next region. If you try to track down Cinder after defeating Hecate, you can abandon your run and return to your camp, as you can only earn one Cinder per run and only Hecate drops them.
For those who played the first Hades, Cinder works a bit like Titan's Blood and the other boss rewards, except you don't need to equip a different weapon or weapon aspect to receive Cinder. Instead, Hecate always drops Cinder as a reward, regardless of how many times you defeat her with the same weapon, and the same goes for every boss in Hades 2. It's much easier to find than Shadow.