"Bála by ses?"

Translation: Would you be afraid?

[tompw89]

Why does the accusative reflexive pronoun "se" become "ses" here?

[JanLyko]

When using a reflexive verb in the second-person singular past form and in conditional, the auxiliary verb být (“to be”) is replaced with just -s appended to the reflexive pronoun se, si. The full form “jsi se”, “jsi si” is proscribed as hypercorrect.