Ever since the Golden Ages, the Guardians' had fought to protect the children of Earth. They vowed to stop Pitch, the Nightmare King, from harming the children from his nightmares.
The battle had went on for centuries with no sign of letting up.
As years went on, the constant battles between the two forces put a huge toll on the children's belief. The Guardians, rather they knew it or not, were neglecting their duties.
This did not disturb the Nightmare King though. He hated the guardians and MiM for the destruction and loss it had caused him.
Seeing his family vanquish at the hands of the Guardians, leaving him with a empty feeling inside, made him angry. Trying to stop him from spreading fear made him just as angry.
Thinking that the children needed just Wonder and Dreams and Hope and little fairies that collect teeth, sickened him.
It sickened him that after all these years they think they could get rid of fear. But what made him hate them worst of all was the fact of what happened to his younger brother.
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