The fifth edition of Dungeons & Dragons breaks abilities down into those usable at-will and those which recover their uses after a short or long rest. Naturally, the more powerful the ability, the less frequently characters can use it. A part of game-play is resources management: Should you use your big long rest ability now or wait until a more opportune moment? Do you know when the next opportunity for a short rest will come, or is the next encounter arriving on the heels of the current one, without a break in between?
Ordinarily, if a character is out of uses of an ability that’s it. It is no longer available to them until they have completed the necessary rest. However, there are times when it may be dramatically appropriate for characters to push their abilities beyond their normal limits, when they really need the use of an ability they’ve already expended. Fortunately, the fifth edition system offers a useful resource in that regard: Exhaustion.
Detailed in appendix A: Conditions of the Player’s Handbook, exhaustion is a condition that measures increasing levels of fatigue as characters expend their physical and mental resources. Each level of exhaustion imposes increasing penalties on the character, and finishing a long rest reduces a character’s exhaustion level by 1, conveniently making a level of exhaustion roughly equal to a long rest in “value.” This sets up the potential for the following variant:
Exertion. You draw on inner reserves of strength and determination to accomplish something. As a bonus action, gain 1 level of exhaustion, and choose from either gaining advantage on your next ability check, attack test, or saving throw, or regaining one use of an ability regained by completing a short or long rest. You do not gain any of the other benefits of rest from exertion.
One use of exertion is largely “free” as the character will remove the level of exhaustion after completing their next long rest, although they’ll have to deal with disadvantage on ability checks until then. Uses beyond the first have diminishing returns, since the exhaustion will take multiple long rests to recover. By level 5 exhaustion, the character is spent, speed reduced to 0 and unable to do much more than collapse against something and rest. Exertion past that point to level 6 means death, although the GM should consider delaying that penalty until the end of the character’s turn, allowing them one last glorious effort.
Exertion and Spell Slots. Exertion specifies “one use” of an ability that recovers after resting. This is incompatible with the recovery of spell slots, all of which recover from a long rest. On the other hand, recovering just one spell slot seems a poor trade-off for a level of exhaustion, so it’s recommended that spellcasters be allowed to recover up to half their class level (rounded down) in spell slots from exertion, with no spell slot greater than 6th level, or recover a single spell slot of 7th level or greater at a cost of 1 level of exhaustion for a 7th-level slot, 2 levels for an 8th-level slot, and 3 levels for a 9th-level slot. This is a version of the Natural Recovery and Arcane Recovery abilities of druids and wizards, respectively, but available to any spell-casting character through exertion (whereas wizards and circle of the land druids can still use it simply by taking a short rest).
Game Masters can fine-tune the requirements of exertion to suit the game, possibly increasing its cost to 2 levels of exhaustion per use, rather than 1, effectively limiting it to two uses (since a third would be 6 levels of exhaustion and death) and eliminating the “free” aspect of one use, since it would take a minimum of two long rests to fully recover from a use of exertion. An even more limited version would require the character to have inspiration in order to use exertion: They expend their inspiration and immediately regain one use of an ability that recovers after a short or long rest. In this case, the GM may or may not also require that the character gain a level of exhaustion. See Acting on Inspiration for more on this notion.