I got into a discussion over Discord this morning about whether the Dream spell could be used to kill someone. The claim was, if I remember correctly, that it would only take 6 days/nights because that's how many level of exhaustion it takes until exhaustion leads to 'Death'.
Exhaustion table from the DMG, as sent to me in Discord, transcribed below
Level | Effect |
---|---|
1 | Disadvantage on ability checks |
2 | Speed halved |
3 | Disadvantage on Attack rolls and Saving throws |
4 | Hit point maximum halved |
5 | Speed reduced to 0 |
6 | Death |
And, from the spell text:
On a failed save, echoes of the phantasmal monstrosity spawn a nightmare that lasts the duration of the target's sleep and prevents the target from gaining any benefit from that rest.
Emphasis mine.
It turned out they were also assuming the alternative rule from Xanathar's Guide to Everything, and sent this picture of the rules:
GOING WITHOUT A LONG REST
A long rest is never mandatory, but going without sleep does have its consequences. If you want to account the effects of sleep deprivation on characters and creatures, use these rules.
Whenever you end a 24-hour period without finishing a long rest, you must succeed on a DC 10 Constitution saving throw or suffer one level of exhaustion.
It becomes harder to fight off exhaustion if you stay awake for multiple days. After the first 24 hours, the DC increases by 5 for each consecutive 24-hour period without a long rest. The DC resets to 10 when you finish a long rest.
(I've included images in case they sent a wrong version/edition, but transcribed where I can.)
Altogether the assumption was that failing to get a long rest for six lots of 24 hours would lead to six levels of exhaustion and thus death.
Given that the spell Dream just replaces a steadily increasing Constitution saving throw (DC 10 + 5 for each 24hr without a long rest) for the flat Wisdom saving throw against a spell caster's DC, is it possible to calculate how long a character with a 10 in Constitution and a 10 in Wisdom would survive in either scenario (with or without Dream interruption)? It won't be just \$P(\text{fail save})^6\$ because of the third level of exhaustion leading to disadvantage on saving throws and/or increasing DC in the case of willingly not sleeping.
In the case of using Dream you can assume the caster can't impose disadvantage, and has a \$\text{Spell DC} = 8 + 4 (\text{proficiency}) + 5 (\text{ability score}) = 17\$. Also assume there're no 'tricks' loopholes like granting the wouldbe sleeper bonuses to saving throw bonuses, or using spell or abilities to remove exhaustion or making sleeping unnecessary (e.g. being an Elf, Warforged, or high level monk or having taken Aspect of the Moon as a PAct of the Tome warlock...)
That is, voluntarily or otherwise, how long can my character survive without sleep?