Vodyanoi are moody fey spirits of rivers and lakes. They are able to control their body of water with remarkable precision, raising or lowering its level, and making fish abundant or absent as is their wont. Villages near a vodyanoi-infested body of water seek to propiate the creature with offerings of fish and cattle, and this tenative alliance often works, but vodyanoi can be mercurial and tempermental.
Moody and wily, vodyanoi don't often consider human life something of much value -- though they don't go out of their way to kill, either. They are easily offended, and aren't above ambushing someone beside the river in a fit of pique. They grab and drown those who have offended them, control the water of their river to wash others in and create difficult terrain. When truly threatened, they are capable of summoning a school of magical fish to cover their escape.