In answering this question one needs to have in mind many assumptions, most importantly your expected number of encounters.
- Based on guidelines in the DMG, I assume about 100 encounters over a character's full career (1-20)
- I divide those between <hard and >hard at about a 2:1 ratio
- The former I measure about a 0.8% risk of character death, and the latter about 8%
- My rule for replacement characters is they are rolled at one level below the lowest level survivor
- With that rule, we tend to top out at tier 3 e.g. level 15 characters at most
Given those values, a party of four will likely experience about 50 deaths over their career.
- With revival magic, many of those are survived
- In my world, revival costs several hundred to several thousand gold pieces, depending on the spell dictated by the manner of death
- I reduce the DMG guideline scaling of treasure hoards, so parties are not as cash rich in my campaign
I find that, with
less than 1% chance for a character to die in most encounters, parties need about 30 lives over the full course of a campaign. Historically that has amounted to about a dozen lives after revival magic, i.e. 12 characters generated for 4 to survive.
How can a DM use this information? What I have found is that by tracking the number of deaths, and knowing my background assumptions for availability and affordability of revival magic, I have hit a point that players describe as
punishing lethality. That works for my group. I have also found that having a bright line between normal encounter, and a deadly one, has been helpful.
As well as being mindful of the cumulative chance over encounters, and the viability of magical revival, you also need to think about
what is at stake? Our most recent death slew a much-loved level 8 druid. The player will reroll a level 4 character. Their stake - what was swept off the table - was levels 5-8.