You're looking for a oneshot. (a dnd campaing meant to be short and completed in a single session.) The joy of oneshots is that your players are usually not particularly attached to the characters, so a TPK won't necessarily piss everyone off, and you can ramp the difficulty some.
plenty of sources for free (and not free,) oneshots in a variety of the usual places.
DMsGuild has plenty that are free, or 'pay what you want' as a tip, if you feel so inclined.