The Rivers of Blood katana is a powerful, late-game katana that applies massive bleed with its Ash of War ability Corpse Piler.
However, don’t expect to get it any time soon in a playthrough. Rivers of Blood is in the hands of Okina, who invades the player near the Church of Repose deep in the Mountaintops of the Giants.
To so much as reach the Mountaintops, players must go to Leyndell, defeat Morgott, the Omen King, and make their way to the Grand Lift of Rold. From the Mountaintops side of the Grand Lift, it’s a long trek north and east around the mountain. Players will cross an immense but still somehow long, thin bridge past the Zamor Ruins, then up and around the cliff face.
Continuing east, then making their way south beyond the Guardian’s Garrison will see players cross a bridge made of chains. The Church of Repose sits on the southwestern edge of a snowy field of giant corpses. Once near it, Okina invades with Rivers of Blood equipped, ready to murder anyone who comes close to the church. Killing him rewards his mask and the katana.