Save or die is an Old School marker and by way of poison/venom a commonplace.
Some suggestions for devastating toxic effects that aren't just game over.
If you're not already using delayed effect mechanics, then: d6 time units (as appropriate) from the poison/venom being administered until it starts to do something.
Decide whether the save is made at delivery or onset.
Recovery from poisoning could be entirely reliant on medical or magical treatment, or could be dealt with in a similar way to natural recovery from disease.
Until the character receives cure poison or comparable:
- Must make a death save every day or die in their sleep.
- Disadvantage/auto-fail all STR and CON checks (odds/evens).
- Disadvantage/auto-fail all saves (odds/evens).
- No magical/natural healing (odds/evens).
- Lose 1 point from each Ability Score per day (until cured or dead).
- Minimum hit points for class, level and CON.
- If you're exposed to the same poison/venom again, you will die.
- Sickness/Nausea (like a Troglodyte's stench or Giant Centipede's venom).
- Weakness/Fatigue (as a spell or subsystem).
- All your Ability Scores reduced to 3 (or minimum for your system).
- Natural healing rate uses next longest time unit (e.g. day to week, week to month).
- Must make a death save every time you have to roll any other dice.
Retain any non-fatal special effects of a particular toxin, for distinction and flavour.
A fatal poison/venom can have ongoing effects as above, but be ultimately deadly within d6 time units (the shorter, the deadlier) after onset unless properly treated. There's drama here.
In addition, poisoned NPCs and monsters could have disadvantage/auto-fail on Morale/Loyalty checks and saves against fear due to the trauma of thinking they're going to die.
STR or CON minus a flat or random number can also be used as a measure of time units for onset/ fatality/ recovery.