The Sahagin have summoned Leviathan, and the Maelstrom have prepared The Whorleater for the counterattack.
How to Unlock
The Whorleater (Hard) trial is unlocked by the level 50 Main Scenario Quest Lord of the Whorl.
Attack Leviathan until he disappears. Watch and listen for water plumes that appear around the edges of the platform. Don't be next to the water plume. Go as far away from it as you can to avoid getting hit. This will also make the platform tilt, and you will slide across it.
When Leviathan and Leviathan's Tail appear, target each one to look at the buff that it has. Leviathan will have the "Veil of the Whorl" buff, which will reflect damage from ranged attacks, so only hit him with melee attacks. Leviathan's Tail will have the "Mantle of the Whorl" buff, which will reflect damage dealt by magic attacks, so only hit it with physical attacks. The damage is shared between the two, so just focus on hitting the one that is safe for you to hit.
Use the Elemental Converter whenever Leviathan disappears, if it is operable. A message will appear when it becomes operable. It is on the south end of the platform. You need to use it to survive Leviathan's Tidal Wave attack. It can only be used if the Elemental Conversion gauge is at least 30.
Gyre Spumes will appear at the corners of the platform. They will deplete the Elemental Conversion gauge, so kill them as soon as possible.
If any Wavetooth Sahagin appear, try to kill them quickly. They can cast Dreadstorm, which creates puddles on the platform that inflict a Fear status on any players that step in it, causing them to lose control and run around at random. This can cause the player to run back into the puddle, which makes the Fear status countdown timer start again from the beginning. Wavespine Sahagin can appear as well, but they cannot create puddles, so they should only be killed after the Wavetooth Sahagin have been killed.
Leviathan will periodically target a random healer with Waterspout, which damages the targeted player and those near them, so the healers should stay apart, and players should avoid getting too close to them.
Completing the Quest
Defeating Leviathan will complete one of the quest objectives for the Main Scenario Quest Lord of the Whorl.