Subek

For most of the year the subek are a kindly race, advising others and lending their physical and intellectual prowess to local projects. During the flood season, however, subek become violent and territorial, ruthlessly killing and consuming all trespassers.
Riverbank Homes. Subek are crocodile-headed humanoids that dwell along the banks of great rivers. They are tailless, possessing muscular physiques, surprisingly dexterous hands, and a frightening maw of sharp teeth. They are 9 feet tall, average 700 lb., and can live up to 300 years.
During the dry season, subek are friendly, thoughtful scholars, historians, and artisans.
Flood Fever. Subek are well aware of their destructive and violent nature. When the waters rise, they distance themselves from other cultures, warning locals to keep away until the river recedes. Most migrate up or down river to an area with few inhabitants; some even construct underground prisons or cages and pay brave retainers to keep them locked up and fed during their time of savagery.
During flood fever, subek do not recognize friends or colleagues. They discard all trappings of civilization and kill nonsubek creatures indiscriminately. Once the fever clears, they remember nothing of their actions, though they are surrounded by undeniable, grisly reminders.
Keep Their Distance. Despite the danger, subek are tolerated and even prized for their skill as engineers, historians, and teachers. They live on the outskirts of many human towns, maintaining a cautious distance from their neighbors. Subek marriage is pragmatic; they live with a mate long enough to foster a single egg and raise the hatchling for a decade before parting ways.
Subek scholars and oracles debate their duality. Some believe it to be an ancient curse, a shared ancestry with northern trolls, or some loathsome and primitive part of their soul exerting control. A rare few—shamans and oracles, mostly—embrace their duality and choose to live year-round in remote regions far from civilization.