Indomitable (3 uses) is a Fighter feature gained at level 17 in D&D 5e. Beginning at 9th level, you can reroll a saving throw that you fail.
Beginning at 9th level, you can reroll a saving throw that you fail. If you do so, you must use the new roll, and you can't use this feature again until you finish a long rest. You can use this feature twice between long rests starting at 13th level and three times between long rests starting at 17th level.