A Hawkman guides some cavemen in worship of the giant idol...
Olmec is the final boss of Spelunky. It is located in Olmec's Chamber, at the end of the Temple on level 16.
Upon entering the final level, the Spelunker sees a Hawkman leading a group of cavemen to worship the giant figure, which is encrusted with dirt and covered in vines from years of lying dormant. Upon sensing the intruder, the ancient totem awakens and rises into the air.
The Hawkman is knocked unconscious as the Cavemen attempt to flee, the slowest of which is crushed beneath Olmec as it smashes down. The Giant figure then rises into the air again and approaches the Spelunker.
By defeating Olmec, the Spelunker earns the Giant Idol and wins the game.
Behaviour
...which promptly goes homicidally insane.
Olmec is very similar to the final boss of Super Mario Bros. 3.
It cannot be damaged by conventional attacks, and touching it anywhere while it's moving will kill you instantly. There is one safe spot on the boss - the very top of the head.
To attack, Olmec rises into the air and hovers briefly before landing. If the Spelunker is underneath it during the hover, it will smash down. This destroys the blocks it lands on, and causes clusters of yellow energy to shoot from its gem that transform into enemies when they hit the ground.
Strategy
The only way to destroy Olmec is to force it to drop into the lava pool that spans the bottom of the level, beneath the ten layers of brick that make up the floor.
This is possible to do simply by running underneath it, but is much easier if you thin the floor out with bombs or by digging with a mattock.
Although touching Olmec while it's moving is dangerous, if you can climb on top of it while it is stationary you can ride the figure as it jumps, allowing you to reach the treasure in the upper parts of the level without the use of a rope.
When fighting Olmec, remember that defeating the boss only causes a door to appear at the far right side of the level. You still need to get to it in order to win the game, so be sure you have some means of reaching the exit once the boss smashed has through most of the floor.
The key element in this fight is to remember to have a place to go when Olmec jumps, aside from under him.
Olmec moves very slowly horizontally, only one square per 'jump'. You can gain time to dig a pit trap by moving to one side of the level and waiting for him to follow you, then running back to the other side, digging a pit through to the lava there using bombs. Stand on the far side of your trap and the Giant Head will have no choice but to follow and fall in.
Inspiration
The final boss is called "Olmec" in some places in the source code, indicating the boss is inspired by the colossal heads of the Olmec civilization.
Gallery
A typical example of how Olmec's chamber generates