Purpose of the 3rd Plague
And the third angel poured out his vial upon the rivers and fountains of waters;
and they became blood. For they have shed the blood of saints and prophets,
and thou hast given them blood to drink; for they are worthy.
Revelation 16:4,6
And in her was found the blood of prophets, and of saints,
and of all that were slain upon the earth.
Revelation 18:24