。
I promise to come here tomorrow.
Literal
Tomorrow here [to-へ] come [nominalizer-こと] [object-を] promise.
こと here nominalizes the verb phrase 来る ('come') so that 約束する can take it as an object: 'promise the act of coming.' Distinct from の nominalizer — こと is preferred for formal/abstract nominalization (promises, decisions, laws), while の is for concrete or experiential ones.