The succubus has an ability called Charm. Its description states that a suicidal command results in the charmed creature repeating the saving throw to resist the command:
If the target suffers any harm or receives a suicidal command, it can repeat the saving throw, ending the effect on a success.
But what is considered a suicidal command? Is it only a command that kills you 100% of the time? Is it any command that damages you? How about indirect damage?
Jumping off a 1000-foot cliff sounds pretty suicidal, but at level 20, most characters could survive that easily, since falling damage caps at 20d6. There is almost nothing in the game that could consistently one-shot a level 20 character, so does that mean nothing is considered suicidal to them at full health?
I have considered maybe a percentage damage. 50% of max health could be considered suicidal, but the problem is that any set percentage would be arbitrary, and that sounds very much like a hidden rule, which D&D tries to avoid. Any damage could mean lethal consequences if you only have 6 HP. Every hit point is extremely valuable, so unless the percentage shifts according to max HP and other weird stuff, this version wouldn't really make sense.
Maybe if it does kill you, it is considered suicidal? The problem is you cannot know whether something kills you until you roll the dice, and you cannot roll the dice until it is your turn, since plenty of stuff could happen in between getting the command during the succubus' turn and yours. And since a successful save means you resist the charm, and the saving throw is supposed to happen when you receive the command, this isn't really possible.
The best option I can find is counting all damage you do to yourself through environment or actions as "suicidal".
Are there any official rules on how a "suicidal command" is defined?