Cycle tides
An exceptional large movement in sea levels that can trigger destructive waves or cause deep sea beasts to come to the surface.
Manifestation
Once evert 309 years the level of the sea can undergo a dramtic change as the balance between the plane of water and the material plane shifts. This is seen all over the world as tides reach unusually high or low levels. Sometimes the water levels change for an extended period leaving coatal towns flooded or fishing fleets left stuck far from the sea. On several occassions the temperature of the new water has altered the weather across much of the world, with sudden changes in levels of snowfall, rain, or drought. While this did correct after a few years the short term effects could be devastating until the Wind Guides were created.
Even if a destructive wave isn't created the influx or removal of water can affect the habitat of deep sea beasts which move closer to the surface in order to find conditions more to their liking. This can lead to an increase in attacks on shipping, and attacks on coastal or merfolk settlements.
Depending on the exact timing of the shift in the balance and the position of ther sun and move these cycle tides can result in more destructive effects. One of the most destructivr of these aftereffects are the large waves that can be set moving across the ocean. Far more powerful than a wave triggered by an earth tremour these devastating waves can reach further inland and cause destruction over a wider area.
A Predictable disaster?
During the second age a way was developed to predict the exact date and nature of this tides. However the knowledge and tools required were lost to all but the elves
During the second age a way was developed to predict the exact date and nature of this tides. However the knowledge and tools required were lost to all but the elves
The cycle referenced in the name is the movement of the material plane relative to the Other Planes. 
As the middle of the period of water is reached more water flows into the seas causing them to rise. Similarly when the middle of the period of fire is reached the flow is reversed and the level of the seas drops.
Cycle of planes by Tanai Cuinsear
Love this concept It is far too common seeing a static relation between the planes in fantasy worlds.
World Ember came and went. Check out Freelands!
It’s proved a useful concept for creating disasters (this is far from the worst) and world altering events. It’s fun to see the player characters know something is coming and their attempts to convince the power groups to prepare.