Flintslate is a tool in Hollow Knight: Silksong that imbues Hornet's Silk Skills with fire damage, increasing her attack power by temporarily super- heating the needle's blade. It also provides some protection against cold damage. Flintslate is found in the Deep Docks area behind a locked door that requires a Simple Key to open. The key can be bought from a merchant in Bone Marrow for 500 Rosaries. After unlocking the door, players must navigate through enemies and platforms to reach the bench where Flintslate is located. It has three charges and its fire damage burns enemies on hit but lasts only briefly, so timing its activation is important for maximum effect.
If further detailed information is desired about "slate" or "Silksong" more broadly beyond Flintslate, let me know. However, the main context here is Flintslate as a key tool in Hollow Knight: Silksong.
