Fall of the Aquatic City

For centuries your people have guarded this place, deep under the waves. The city of Tramaria has held the heart of the Goddess Una in its depths. That was, until a vision of ruin was imparted to the High Priestess by Una - Ruin that could only be averted if Una's children went out into the wider world and collected Eight Artifacts and brought them back to the city.
After many months of searching, the heroes that the city sent forth succeded and ten days ago the eight artifacts were returned. It was only then we learned that for the calamity to be forestalled, an ancient ritual must be performed and the cost would be the city and anyone left in it. Five days ago, the Elders evacuated most of the city, but some guardians would need to remain and protect the ritual chamber from them. Those who would bring ruin to the Whale. You are such a guardian, sworn to protect the ritual though you know its successful completion will cost you your life.
The time has come and the hour has drawn close - you can hear the thrum of energy as the ritual begins deep in the city. They have begun to converge - they stalk the silent halls of the city, intent upon breaching the chamber and stopping the ritual. It's up to you and your remaining guardians to stall them long enough for the ritual to be completed. The elders and Una yourself have entrusted this task to you, everything hinges on this moment. Steel yourselves and take the fight to the enemy!
Additionally, even though the heroes are doing something heroic and know they will meet their ends one way or another, it still fits very well in the Tragic Horror genre. How will our heroes meet their ends? What dark secrets will bubble forth before the end? Will they stick together, or be corrupted? Will they turn on each other? Try to flee the city?
Places of Note: The Kelp Beds, Nursery, Seahorse Stables, Grand Ballroom, Market Square, The Ritual Chamber
Goal: Hold the invaders back until the ritual can be completed, killing everyone left in the city.