End spike

Obsidian Pillars (also known as Obsidian Spikes) are generated structures in The End that are made of Obsidian. Obsidian Pillars can not be destroyed by the Ender Dragon, as they consist of Obsidian. Rather, the Dragon will simply pass through them. They are approximately circular, with a diameter of 3 to 9 blocks, and a height of anywhere from 6 to 37 blocks. Each chunk has a 20% chance of generating an obsidian pillar. Larger pillars can be 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.

The narrowest pillars are square in shape:

Usage
A single Ender Crystal spawns on top of each pillar, which heals the Ender Dragon.

Obsidian pillars provide an excellent source of obsidian.