The Thalassara

The Thalassara is a gigantic maelstrom located in the middle of Fulgium. It is the second of the Seven Natural Wonders of Anu and its great and chaotic tides and streams are responsible for the global currents crisscrossing the oceans of Anu. 
In the elder days, the Thalassara arrived on Anu as a colossal meteorite. When it impacted the planet, it immediately began to let out a seemingly endless torrent of water into the canyons of the planet. It was the Thalassara that cooled the fiery hell of primordial Anu down and with the creation of Water Elementals ended the sole supremacy of the Fire Elementals.
Eventually, the streams flowing out of the Thalassara slowed, the pressure from the great oceans pressing down on it. But it never reached a stable equilibrium. Instead, the Thalassara goes through cycles. The so called waning and waxing cycles. 
In the waning cycles, the Thalassara absorbs water and it turns into a colossal whirlpool, with most oceanic currents flowing towards the Thalassara. 
In waxing cycles, the Thalassara ejects water and depending on the strength of the cycle it might be seen as a slightly unsteady surface or as a colossal mound of water, a constant giant tidal wave reaching many meters or hundreds of meters above the regular sea level. During a waxing cycle most oceanic currents revers, flowing away from the Thalassara.
These cycles usually change every few years but can range from just a short few months all the way up to many decades. Depending on the strength and duration of a cycle, the sea levels can drastically change. For most regular cycles the change in sea level is almost imperceptible, but for the strongest and longest cycles global sea levels can rise up or fall many meters.
The Thalassara and its surrounding ocean became the home of many underwater Beastfolk races. These races usually worshipped the local Water Elementals. These underwater states, usually organised in theocracies or religious monarchies lived largely separate from the surface dwellers and so far below the surface of the ocean where the sun could reach them, they measured their time and ages by the change of the cycle, with each cycle having a specific meaning. The waning cycles were times of plenty, where fish and minerals carried by oceanic currents flowed into great nets and could be harvested with ease. The waning cycles were seasons of celebration, festival, construction, growth but also conflict and war. The waxing cycles were times of little, with food and resources being carried away by the oceanic currents. The merfolk had to rely on their stores and send out large expeditions on great journeys to find and bring back food and resources. The waxing cycles were seasons of piety, reflection, community and diplomacy.
The the Old Empire and the Imperials first arrived on the shores surrounding the Thalassara, them and the merfolk below didn't know of each others existence. Eventually the imperials began to explore the depths of the Thalassara and the surrounding ocean, building underwater cities as rest stops. It was during this time that the imperials and the merfolk first came into contact and war. At first the merfolk, being more adapted for combat at such depths won with ease and even pushed back the imperials. But the imperials, not bound by the cycles of the Thalassara were able to slowly gain the upper hand through a brutal war of attrition. 
Eventually, the merfolk realms entered a conditional surrender. Uniquely they weren't destroyed and absorbed into the Old Empire, instead becoming vassals, overseen by imperials living in underwater cities above the merfolk cities. Over the ages however, as imperial power, technology and magic ability improved, they began to exert more and more influence on their would be vassals until the merfolk realms had almost no autonomy, vassals in only name they effectively were integrated into the old empire.