Just posted this in the 5e Fighter thread.
But this can easily be accomplished in other systems (eg in Dungeon World this would be a playbook move whereby the Fighter rolls at the beginning of the day for Hold and can spend it throughout the day to either (a) allow an allow to reroll a die or (b) allows the use of a move that demoralizes an enemy).
I'll going to put an addendum to my post upthread and since this is 5th edition that is being talked about here, I'll use that system to express the concept.
One of the most impactful aspects of an athlete who is the apex predator, the absolute pinnacle of the dominance hierarchy (the proverbial king, or queen, of the jungle), is how the rest of the members of that hierarchy are either emboldened by their presence (if they share cause) or they absolutely wilt in their presence (if they are opposed).
Tiger Woods overwhelmingly won on Friday and Saturday. What happened on Sunday? Being caught in his orbit, his competition wilted under that overwhelming task (playing below, often well below, their natural capabilities) time_after time_after time_after time. Tiger wasn't a comeback king. He was an absolute front-runner with his ability to just hit clutch shots when he needed to just completely debilitating his opponents who were absolutely top of the food chain...if not for one Tiger Woods.
Michael Jordan and the Bulls?
Exact same thing. Teams were beaten before they took the court. Overwhelmingly, opposition (guys who, again, were absolute far, far, far end of the distribution of human capability) spoke in reverential, fearful tones of him (while they were in the freaking league together)...never wanting to draw his ire and give him (and through him, his teammates) cause to grind them into absolute dust.
The weight of his presence, the specter of inevitability through him looming:
1) Moralized allies to heights they would be not be capable of otherwise.
2) Demoralized opposition to depths that they otherwise had no business falling to.
How do you accomplish this in 5e?
Simple,
Reskin the Diviner Subclass ability Portent and have it work mechanically in exactly the same way (because that is effectively what happens in our world).