Choose the correct phrase to complete the knight's nervous confession.

Sir Reginald lost the Royal Broadsword in the swamp, and he _____ the King, for His Majesty has a notorious temper.

The correct answer is dare not tell.

"Dare" can act as a marginal modal verb, especially in negative sentences. As a modal, it does not take a third-person "-s" (so "dares" is wrong), it forms the negative by simply adding "not", and it is followed by a bare infinitive (without "to").

Therefore, "he dare not tell" is the only grammatically correct standard form here.