Here's why:
* Metaphor: A metaphor is a figure of speech that compares two unlike things without using words like "like" or "as".
* Comparison: The sentence compares the mighty oak to a king.
* Implied Meaning: It implies that the oak tree is strong, powerful, and dominant in the forest, just like a king is in his kingdom.
The sentence doesn't explicitly say "the oak is *like* a king," it directly states it *is* a king, creating a metaphorical comparison.