exhaustion rules:
-1 to all d20 rolls, -1 to AC, -1 to all DCs
-5 ft speed per 2 levels, round down
max 10 levels, dead if you gain another exhaustion level after 10.
being knocked out gives 1 exhaustion level, this level goes away after short rest or being healed to 50% of max HP(might be 75% max HP if you want it even more gritty).
if a critical hit lowers you to less then half HP, gain 1 exhaustion level. goes away in the same rules as above.
having no available highest level spell slots as a spellcaster gives you one exhaustion level. removed on short rest.
short rest is 1min long but taking more than your proficiency bonus times per long rest gives one exhaustion level. this can only be removed by long rest.
having 0 HDs left gives 1 exhaustion level, removed when you regain any HD.
taking voluntary exhaustion levels:
if you suffer critical hit, take 1 exhaustion level to turn it to normal hit.
if you fail saving throw, take 1 exhaustion level to re roll the save. Once per short rest.
when you take Dodge action, take 1 exhaustion level to gain benefits of a short rest. No exhaustion levels are removed by this version of Short rest, but it still count as number of Short rest before exhaustion kicks in.
if you make melee hit, take 1 exhaustion level to turn it into critical hit.
Action surge, take 1 exhaustion level to take 1 extra Action on your turn. Once per short rest.
Blood casting: when you cast leveled spell, you can take a number of exhaustion levels equal to spell level. you cast the spell without using spell slot. Cannot be used on spell slots higher than 5th.