Gumball Guardian

The Guardians of the Royal Promise make their first appearances in the episode Slumber Party Panic. They are first seen on opposite sides of Princess Bubblegum's Castle blowing bubbles. The Guardians will freeze time and appear to anyone who breaks a royal promise and destroy them. Usually they hold a (literal) trial by fire to the promise-breaker. However, in Slumber Party Panic, Princess Bubblegum manages to stop the trial by fire because of her royalty, so instead they ask Finn a math question: originally long and very complex (and involving an approximation of pi), but one of the Guardians changes his mind and instead gives the question of what is "two plus two". Finn is terrible at math, yet correctly guesses "four". The guardians' heads then blow up, reversing time to just before they came to grab Finn and the princess. They then go back to their places on the sides of the Candy Palace and begin blowing bubbles again.

Appearance
The Guardians of the Royal Promise are two giant gumball machine men who wear ice cream cones on their heads as hats. Their skin may or may not be pink, as it is possible they could simply be wearing colorful shirts.

Quotes
"Finn the human. You have broken a royal promise, for which the penalty is trial by fire."

"Because you care for the promise-breaker, we will give him a less hard trial. You must now answer math questions."

"Oh wait, wait. I thought of a better one."

"Yes! Two plus Two! Solve it or die."

"Oh. Correct."

Trivia

 * In Princess Bubblegum's lab, there is a dummy Guardian of the Royal Promise. It is seen after the real Guardians leave her and Finn.
 * In Ricardio the Heart Guy, when Finn was spying on Ricardio, one of the Royal Guardians wasn't wearing his cone hat.
 * In Ricardio The Heart Guy, there are two smaller Royal Guardians in the throne room where Ricardio holds Princess Bubblegum hostage. They are seen wearing royal cloaks and blowing horns.