KanjiJLPT N1
Imperial "I"

majestic plural, imperial we

Mnemonic

Under the moon, a sacred deer with golden antlers kneels before the throne. Only when the deer bows does the Emperor rise and declare the Imperial "I"—a word so mighty the moon trembles at its sound. No commoner dares utter it; the deer would strike them down.
Additional thoughts
Picture the moon as the Emperor's glowing backdrop, the deer as his divine guardian. Together they form the exclusive pronoun only an Emperor may speak. The deer enforces the rule—only royalty says 'I' this way.
Quick recall
A moon-lit deer guards the Emperor's exclusive Imperial "I".

Details

The keyword for 朕 is imperial "i". This kanji refers to the exclusive first-person pronoun used solely by the Emperor of Japan (and historically by the Emperor of China) when referring to himself. It carries an air of supreme authority and divine sovereignty, distinguishing the monarch's self-reference from all common forms of "I." Its use by anyone other than the reigning emperor would be considered a grave breach of protocol.
On'yomi
ちん