Just waiting for the first people to show up so we can play our game tonight. Starting right off with a fairly meaty monster; party is already hurting somewhat for hit points, so this is going to be particularly nasty (potential for 40 hp damage if a save isn't made). Sorry I can't say what the monster is; it's part of Ternketh Keep, part of my crowdfunding proposal.
It could kill someone. That's the point.
In my game, that's a lot of tension. The players got sight of the thing at the end of last running, two weeks ago, so they're bound to be ill at ease as they come in tonight; I know that my partner has been on tenterhooks for the last few days, worried that she's going to lose her 4th level henchwoman fighter, Holly. I don't think she will - they've got plenty of healing, poison antidote and both slow poison and death's door as spells if it comes to that. Still, I want the party to worry - so as they come in, I'll wait for just the right moment to express sorrowfully, with concern, "I hope I don't kill someone tonight."
The key is to say it with honest regret; to sell the moment as something that I, as DM, have legitimate reason to be anxious about it. Moreover, the moment it is said IS very important; not as the party has just come in, or as they're working themselves up . . . but in that sweet-spot moment when it is plain they're gathering their courage. When is that? Well, one has to get a feel for these things.
See, they're going to mention it themselves; they're going to mention it to each other; they'll need to, because that is a part of building each other up. It is like the chatter that goes on before the start of a football game, sometimes between plays. This is not the moment; the moment is when this crests.
We must listen when our players talk to each other (if they're not talking to each other about the game, you have party troubles). We want to hear what they say. We won't get into their heads if we don't.