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 - Art of the Inferno
5 Golden Scarabs - Spirit of the Devils
10 Golden Scarabs - Art of the Ice Storm
15 Golden Scarabs - Jewel of the Demon Seal
20 Golden Scarabs - Spirit of the Devils (II)
25 Golden Scarabs - Jewel of the Demon Seal (II)
30 Golden Scarabs - Armlet of Fortune
35 Golden Scarabs - Jewel of the Demon Seal (III)
40 Golden Scarabs - Art of Inazuma
45 Golden Scarabs - Spirit of the Devils (III)
50 Golden Scarabs - Dark Dragon Blade