The Succubus’ charm says that:
One humanoid The Fiend can see within 30 feet of it must succeed on a DC 15 Wisdom saving throw or be magically Charmed for 1 day. The Charmed target obeys the fiend’s verbal or telepathic commands. If the target suffers any harm or receives a suicidal Command, it can repeat the saving throw, ending the effect on a success. If the target successfully saves against the effect, or if the effect on it ends, the target is immune to this fiend’s Charm for the next 24 hours. The Fiend can have only one target Charmed at a time. If it charms another, the effect on the previous target ends.
So we know that the effect ends if the succubus charms another creature, and that the creature can re-attempt to make its saving throw if it takes damage or is given a suicidal command.
What happens if the succubus charms a creature, and then dies shortly after? Does the effect end, or does it continue until the creature takes damage or after 24 hours?