There Can Be Only One (Monkey Knowledge Point)

There Can Be Only One is one of the final upgrades in the Magic Monkey Knowledge tree, introduced in Version 3.0. It gives access to the Vengeful True Sun God while Monkey Knowledge is active. It requires eight magic knowledge points invested, X-Ray Ultra (formerly Strike Down The False) unlocked, and a one-time purchase of 500.

Strategy
To unlock There Can Be Only One with as minimal Monkey Knowledge Points spent as possible, purchase "SUPER Range" and "Heavy Knockback" along with 2 of any other MK above the 4-MK threshold (e.g. "Lingering Magic" and "Hot Magic"), then purchase "Strike Down the False", "X-Ray Ultra", and purchase 2 other MK (e.g. "Mo' Monkey Money", "Magic Tricks"), and then purchase the There Can Be Only One MK.

In order to summon the Vengeful True Sun God, have a Sun Temple sacrifice $50,000+ worth of tower types of all four categories (Primary, Military, Magic, Support), and have The Anti-Bloon and Legend of the Night placed out of range of that Sun Temple. If done correctly, the newly emerged True Sun God will absorb both of the two Super Monkeys and form a dark mechanically constructed True Sun God on top of his temple base. Additionally, 2 or more of the formerly golden planes will turn dark purple during the process.

For more info about effective strategies using the Vengeful True Sun God, see this page.

Trivia

 * The name "There Can Be Only One" is a quote that references the movie Highlander.
 * There is a Special Mission in BTD5 called "There Can Be Only One" where Bloon Circles on Hard Difficulty must be beaten with only one of each tower type.
 * There Can Be Only One is the only MK whose description matches precisely with its name, albeit in sentence form rather than titlecase.