Emphasizes an extreme or unexpected example, stronger than さえ. Often used in negative or surprising contexts.
He couldn't even write his own name.
Even an expert wouldn't understand something like that.
Even a child knows that.
He doesn't even know his own birthday.
This village doesn't even have a hospital.
すら is another formal 'even' particle that can appear in positive sentences too, whereas だに is restricted to negative, literary contexts.
すら is a more formal or literary way to say 'even', while でさえ is used in everyday spoken and written Japanese.
Derived from classical Japanese すら, a compound of す (an old emphatic particle) and ら (a suffix).