The Sea Serpent is a boss from Hero of Sparta 2. It appears twice during the game and is also seen in the opening cutscene.
The first time you fight the sea serpent, it automatically starts off with a focused kill. If you fail the sequence, you will die instantly. The second fight however, you must fight like you would with a normal enemy. Then, when the focused kill icon pops up, you will do the exact same button sequence as you did with the first battle with the monster.