The Sea Serpent is a boss type of enemy that appears in Tomb Raider: Legend. It appears as a serpent-like creature with, strangely a bird type mouth.
After Lara gets out of King Arthur's grave room, the serpent will emerge. To defeat it, the player must shoot the bells and right after the serpent gets distracted, holster the guns and pull the crane so that the the spiked cells falls on her head. when she dies, her corpse provides a jump platform. The player then will slide to the exit. but a Quick-Time Event showing another serpent (maybe the same). Lara will jump over the serpent's head while shooting a lever so that the gate drops and Lara escapes, the serpent will get angry and turn away.
The Sea Serpent is one of the few Bosses that is not ultimately killed by Lara.