From the Introduction:
The Achelos river takes on the color of blood — cinnabar dust is being dumped into the river. Nicolai Molescu, the old alchemist you rescued from the clutches of the Iron Ring, calls once more for your help in stopping the pollution before it poisons both
the human and non-human population of the Achelos region.
River of Blood is an adventure for 3 to 5 Player Characters of levels 3 to 5. It assumes a total of about 16 levels. Wilderness-oriented characters and dwarves are likely to find their skills useful in several encounters. If the party is too weak, Ongah (see Appendix B) can be used as a reinforcement. Overall, successfully completing the River of Blood adventure will get each PC about 1500 XP.