King Saw is the name of the large sword used by Black★Gold Saw.
King Saw is a large black sword with a Golden blade and a long handle. It's hilt is curved so that it resembles a pointed-down cresent.
In the Black★Rock Shooter OVA, it starts as we see a fight between Black★Rock Shooter and Black★Gold Saw. King Saw is used for all the attacks Black★Gold Saw throws at Black★Rock Shooter. Then, in no more than a minute, B★GS uses King Saw to stab B★RS in the stomach, and B★GS leaves her behind, bringing King Saw with her.