Capcom's super hit Monster Hunter Wilds keeps the content coming. A new free update that just dropped brings back the extremely popular Thunder Leviathan Lagriacus. You can read the full extensive list of changes in the update right here. But you came here to find out how to unlock this bad mammajama.
You'll need to be rank 31+, and have completed the Main Mission A World Turned Upside Down. You will also need to do the side mission Forest Doshaguma for Erik in the Scarlet Forest area. To unlock the quest, you'll need to reset the Scarlet Forest, which you can do by resting in the tent or leaving the area and coming back. Once that's done, you'll unlock the Extra Mission 2 "Trembling Blue Currents."
Alma will let you know that Erik wants to talk to you in The Scarlet Forest. Follow the tracks as recommended, and you should run right smack into Lagriacus, might Thunder Leviathan. There are some underwater parts to the fight, where you won't be able to use weapons. The hunt will drop new materials for weapons and new armor sets for you and your beloved Palico as gear and layered options. This one is a little different in that you need to capture Lagriacus and not kill him.
The seasonal update running with this hunt will give you some special rewards too.
After you complete this mission and reach rank 41+, you'll unlock the 8-star tempered version of this hunt. Now get on out there and hunt!