End spike

General Info
Obsidian Pillars (also known as Obsidian Spikes) are generated structures in The End that are made of Obsidian. They are quite large, with the smallest being 2x2 blocks with a height of 20 blocks. Larger pillars can be 7x7 with a height of 64 blocks, making them suitable for hollowing out as buildings (though the Ender Dragon can still pass through them and injure the player).

Obsidian Pillars do offer some protection in The End, as they can prevent Endermen from sneaking up behind you, and the Ender Dragon generally attempts to avoid flying through them. Fighting from the top of a pillar does prevent Endermen from attacking you, but the Ender Dragon's knockback effect means you are better off fighting from the ground.

Usage
A single Ender Crystal spawns on top of each pillar, which heal the Ender Dragon. When these Ender Crystals are destroyed a Bedrock block will replace the Ender Crystal. Note that the blast radius of an Ender Crystal is always larger than the radius of the pillar, so there is no completely safe way to destroy a crystal while standing on top of the pillar, however, sneaking while on the pillar and destroying the crystal will at least protect you from being blown off the pillar, and wearing a full suit of diamond armor will minimize the explosion damage. If you destroy an Ender Crystal while the Enderdragon is healing from it, the Enderdragon takes damage. This is a quick way to kill the Enderdragon. To get on top of a pillar you have to build a ladder, which is dangerous, because the Ender dragon could destroy the ladder. This, luckily, is rather rare, because the Ender dragon tries to avoid the pillars. The ladder also comes in handy after destruction of the Ender Crystals, because you can jump off the pillar and then slow your fall with the ladder. Another option is to dig your way up. This is not recommended, as mining obsidian is time consuming.

History
Instead of destroying it, the Ender Dragon will fly right through an Obsidian Pillar.
 * Obsidian Pillars first appeared in 1.9 Pre-release 4 along with the introduction of The End. When Ender Crystals were introduced in 1.9 Pre-release 6 they were placed on top of the pillars.