Well, that's Three

 Last night, in the Ravnica game I'm running, the party was assaulting the facility at which a blue dragon named Zagothar, who was the Izzet representative in a plane-spanning conspiracy to bring the Phyrexians to Ravnica, has been constructing a Darksteel Colossus to assault the city and distract from the other conspirators' activities. (So far, the party has defeated the Simic and Rakdos members of the conspiracy.)

In a grueling fight within the central building of Zagothar's floating factory/laboratory, we picked up in the middle of combat from the previous session and the party finally eliminated the two iron golems they had been in combat with (among other things - its a very climactic fight.)

Seeing that the sequence to launch the Colossus (one that the party has just discovered is only one of two,) the artificer ran following signs labeled "Master Control" that pointed them toward the large shaft in which the now-absent colossus had been assembled.

It was there that Zagothar was waiting, and while the artificer survived (barely) a blast of Zagothar's lightning breath, they were subsequently buried by a pile of fallen scaffolding (one of the dragon's lair actions). The artificer had formed a very close bond with the goblin fighter (a member of the Boros Legion,) who, despite being only at 2 health, charged in (the artificer's flight spell and the fighter's Boros-charm-activated haste spell were both active, so they were able to climb the 85 feet to the dragon with ease.)

The dragon then used his three legendary actions that round to smack the goblin, first knocking them unconscious and then killing them with two additional strikes.

While the party took this in good sport, and I don't think I was unfair here (this is a seriously evil boss who isn't going to miss a chance to take out a party member permanently,) I gotta say it didn't feel great.

In the past, I've erred strongly on the side of giving player characters a chance to survive. For one, I've tended to have monsters ignore unconscious people and go after the conscious ones - which has a certain logic to it, as they're still threats.

But I think that, given that the players are not only in tier 3, but also facing a major villain of the campaign, I couldn't just let things slide. I don't know precisely what the chance is for a character to survive on a full set of death saves (it's a little better than 50%, given that you have an 11/20 chance to succeed, and a natural 20 brings you to 1 hit point, though a nat 1 gives you two fails,) which likely would have been what happened to the fighter given that they were so far away (vertically) from the rest of the party.

I also think I need to forgive myself, because this wasn't really a question of tuned fights. Yes, the golems were a huge challenge for the party, but one that I was confident they could handle. I did not force them to run into the dragon fight while still fighting the golems.

In fact, I made a couple of assumptions: that the artificer would be able to cast Fly on the Barbarian to get a tank up in the dragon's face, and also that the party would be able to take a couple minutes before entering the shaft and "aggroing" the dragon, allowing the Cleric to do a prayer of healing, or for people to drink healing potions and take a breath before the final fight.

The massive golem fight was meant to drain health and spell slots so that the dragon would be a bit scarier, and in fact, a major boss fight carrying the threat of death is something that I'd hoped for.

And they're level 11. They have a grave cleric. They might not be able to get the fighter resurrected within Revivify's 1-minute window (in fact, if I were better about keeping track of rounds, I think some spells might have expired in this pretty lengthy fight) but they will certainly have the opportunity to get them alive again.

Now, however, the big question I have for myself is if I want the stress of this event to cause one of the involved characters' planeswalker spark to ignite. I'd intended to save that for higher levels, but this feels like a big enough moment to do this.

So it'll either be when the artificer learns that they got their sibling killed or it'll be the fighter's shock upon returning from the dead. Either way, one of them might be going on a solo adventure on a random plane!