Overexertion (5e Variant Rule)
From D&D Wiki
These rules were created when I realized that constitution had a limited active power inside the game and wanted to offer a greater reward to players who chose to put points in it.
When times are tough, and your players are struggling against the odds, you may offer them the option to overexert themselves. The effect of choosing to overexert yourself could have a variety of benefits;
- Doubling an ability score modifier or attack/damage roll
- Allowing a re-roll
- Taking an extra action
- Running that little bit further/faster (up to Half of the character's movement speed)
- Maintain a Concentration Spell Re-Roll
This has to be a physical act or one that requires a force of will, such as striking an opponent or casting a spell.
After the action is complete the player must then make a Constitution Saving Throw with a DC = Number of times they've overexerted themselves ×5. This DC is reduced by 5 after taking a short rest, or reduced back to 0 after a long rest. On a failure the player receives 1 level of Exhaustion. (To the DM's discretion, if the action they performed through overexertian was particularly significant, they may be unable to perform any further overexertions until they have completed some sort of rest.)