A picture of the Broken Nichirin Item in your inventory.
Broken Nichirins are drops obtained when defeating normal Slayer NPCs. They are not a guaranteed dropped, but there is a high chance of them dropping. Currently Broken Nichirins are used to gain the trust of Muzan (requiring 30 Broken Nichirins) and to obtain the Greatsword (requiring 100 Broken Nichirins). Broken Nichirins can also be sold for 30 Yen (not if you are a human) per Nichirin to Omah in the Demon Purgatory. Broken Nichirins can be obtained as an Slayer, but has no known uses other than a mission later in the game. Broken Nichirins can be dropped from your inventory, allowed for trading.