noun / verb
vow
VOW/vˈaʊ/
A solemn promise or the act of making one.
The partners join hands as they exchange a wedding vow.

Etymology
Latin votum named a solemn pledge, especially a promise to a god. English vow likewise had a religious application before broadening to a solemn promise more generally.
In context
when he heard the decision of the Court he made at that moment a solemn vow to deliver him and then looked around for the means after many fruitless attempts to gain admittance to the prison