Ashes of Darkness are mostly used as quest items on Chapter 2: Strange Noises (General Van Handel: Lost Soul - Examination). The quest requires 3 Ashes of Darkness.
You can get one Ashes of Darkness per 25 floors completed in the Endless Tower.
Another use of Ashes of Darkness is that it can be used to warp to floors 26, 51, and 76 of the Endless Tower. This is useful for farming The Collector Path's quest items, namely Talon of Griffon (7048) and Three-Headed Dragon's Head (7443), which can be found on floors 79, 83, and 86.
|26||1 Ashes of Darkness|
|51||2 Ashes of Darkness|
|76||3 Ashes of Darkness|
Ashes of Darkness are account bound, meaning they are non-transferable between accounts via guild storage, vending, dropping, trading, etc. However, you can put it into account @storage to transfer it between characters of the same account.