I see several potential traps here:
1. Killing the whole party with overwhelming force, even if they've set themselves up for it, is a lousy way to end a campaign.
2. Beating down the party to zero, but then having the bad guys take them alive, can easily feel like a DM taking pity on the party.
3. Declaring that the PCs were captured by fiat, without a chance to react, rubs players really raw.
What I would do is have the bad guys come through the circle in force and hit the whole party with psionic stun attacks. Minions rush to bind and gag PCs who fail their saves. By leading off this way, you make it clear that their goal from the start is capture, so if somebody does end up getting beat down to zero, it won't feel like DM charity to let them live.
Now, I don't believe in combat theater--if you pick up dice, there should be a possibility of multiple outcomes. But nobody said those outcomes had to include "PCs kill all the monsters and win." PCs who make their initial saves can flee, and if they do, they should have a high chance to escape. Then they can set to work rescuing their companions. If they try to fight, they get beat down and captured with the rest, and now the party must escape without outside help.
Of course, all this does depend on you being able to think up a plausible reason for the bad guys to want the PCs alive. If not, I would instead opt for a staged attack. The teleportation circle flares up, giving the PCs one round of warning, and then the first wave of monsters comes through. Then it flares again, heralding the second wave... and the third... and so on. This gives the players time to realize that they are going to be overwhelmed and that they need to GTFO. If they insist on fighting to the death, well, so be it.