As you find Golden Scarabs in the game, Muramasa will reward you with the following items corresponding to how many you've found:

1 Golden Scarab - Smoke Bomb
5 Golden Scarabs - Elixir of Spiritual Life
10 Golden Scarabs - Great Spirit Elixir
20 Golden Scarabs - Great Devil Elixir
25 Golden Scarabs - Smoke Bomb (II)
30 Golden Scarabs - Ayane's Ration
35 Golden Scarabs - Fragrance of Dayflower
40 Golden Scarabs - Ayane's Ration Bundle
45 Golden Scarabs - Fragrance Hydrangea
49 Golden Scarabs - Plasma Saber MkII
50 Golden Scarabs - Dark Dragon Blade