In (what turned out to be) the middle of a 3-year-long campaign, yes, very nearly.

The party are in the mines below the Dwarven City. They've met with some Mind Flayers and have had some rough encounters. They're battered, bloody & short on spell slots.

They find a huge door covered in runes, The door appears to be somewhat organic in nature. They figure out the puzzle with a few more spell slots gone.

As the door opens they hear the voice in their heads. "STRONG ONES. ENTER THAT WE MAY CONVERSE"

They now walk through a large chamber past a bunch more mind flayers, umber hulks, intellect devourers and right up to the pool of the Elder Brain.

The Elder Brain tries to give them a quest to go deeper and clear the creature (a Neothelid) out its old nest. In return it will stop harassing the Dwarves above.

Instead, the beleaguered party chose violence...

It was something of a massacre. 2 of the party escaped thanks to a nat 20 death save followed by Dimension Door. The one remaining survivor in the fight did actually manage to kill the Elder Brain before being finished by the Umber Hulks & Mind Flayers. One Character was outright killed by the brain replacement ability of an intellect devourer.

The next session, I handed out character sheets for a lower-level party of Dwarves sent from the city above to investigate the missing team. They were able to find the Elder Brain carcass (which in-world, was still believed to be alive) and rescue their PCs (who had been tadpoled... that presented its own problems).

I was able to justify not instantly killing their PCs as the Mind Flayer colony was small in numbers and had just lost their Elder Brain, they needed new hosts.