X-Bow

"The X-Bow shoots mystical bolts with terrifying power. Load it with Elixir and the X-Bow works automagically. You can set it to target ground units at long ranges, or all targets at reduced range."

Summary
The X-Bow was introduced in the Halloween 2012 update along with the level 9 town hall. It is unique in two ways. First it has to be loaded with elixir in order to function. It holds 2000 rounds and costs 50,000 elixir to fill (25 elixir per round). Lastly it can be configured to target only ground units at a longer range or both ground and air units at a shorter range.

Supercell posted a teaser video on their Facebook page prior to the update's release:

X-Bow Teaser Video

Trivia

 * The X-Bow has only 1 level, and cannot be upgraded.
 * It is the most expensive defence.