In D&D 5e there are enemy vampires that can bite a human-like person to create vampire spawn.
If the players kill the vampire, what happens with the vampire spawn that were created by that vampire? Do they die, return to full life, stay vampires, or something else?
MM p. 253:
Born from Death. Most of a vampire's victims become vampire spawn- ravenous creatures with a vampire's hunger for blood, but under the control of the vampire that created them. If a true vampire allows a spawn to draw blood from its own body, the spawn transforms into a true vampire no longer under its master's control. Few vampires are willing to relinquish their control in this manner. Vampire spawn become free-willed when their creator dies.
In the vampire entry in the monster manual, the section on vampire spawn states:
Vampire spawn become free-willed when their creator dies.
In particular, in Curse of Strahd, the aftermath section also states that (spoilers, emphasis mine):
Upon his death, Strahd's vampire spawn are freed from his control, and each seeks a new destiny. Escher, in particular, leaves the realm, in search of new experiences and a way to become a vampire lord himself.
This further suggests that spawn would remain undead even after their creator's death.
"A humanoid slain [by a vampire's bite] and then buried in the ground rises the following night as a vampire spawn under the vampire's control."
That suggests to me that vampire spawn would not just come back as human (or elf, etc) after the death of their 'master'. This page states that the process for a spawn to become a full vampire involves:
[..]a true vampire allows a spawn to draw blood from its own body, the spawn transforms into a true vampire no longer under its master's control.
To me, the RAW reads as the vampire is still a vampire spawn, just not under anyone's control. But it is murky; if you are the DM and it is a homegame, you could do any of those options above, and it would likely match with the flavour of the vampire.
