Below is a walkthrough for the Twenty Thousand Years Under the Sea side quest in Tiny Tina’s Wonderlands. To obtain this quest, speak with Oran in Wargtooth Shallows.
Follow Oran
After you speak with him, follow Oran to a temple.
Collect Yarah’s voice boxes
Travel to each of the 5 objectives and defeat the enemies there to receive the voice boxes.
Place Yarah’s voice boxes
After you’ve collected all the boxes, head to the center of the temple and place the boxes on the pedestals.
Defeat Grissnissak
After you place the boxes, Grissnissak will spawn in the room. Defeat it and collect Yarah’s voice that was dropped by Grissnissak.
Follow Oran
Follow Oran to a wall. After he opens it, continue following him through it. He’ll stop at a block. Place Yarah’s voice where indicated by the green ball. Afterwards, the quest will complete.