So I’m playing the PS4 version and like a lot of people really lovin’ the game. I’m almost finished with Act I, so I’ve got a long way to go. Anyway, in the fight against Raedric and his gang, I lost several times, even with a party of six at level 5.
I know, I know, that’s pretty sad. Here’s what kept happening - I’m doing ok and then sometime happens during the fight, I can no longer select one of my characters. Their portrait is red, but they still have plenty of health. Then it’s two characters, three, then four. I get to a point where I literally can’t select anyone during the pause screen.

After this happened three or four times, I finally realized (took me long enough) the two arch mages were probably putting some kind of debuff on my party members...paralysis, dazed, or something. So, I start the fight a fifth time and make sure to take out the two arch mages first - no problem, their dead. Guess what, this stupid crap happens again. By the end of the fight, my main character was the only one left that I could control. Somehow I won the fight. But here’s the thing; this sort of thing never happened before this fight and now it’s happening over and over again. It’s gotten to a point where every fight ends up the same way - with one or two party members that I’m still able to control...everyone else, unselectable.

I thought I had a good grip on the combat mechanics, but now I’m completely lost. I have no idea why this is continuing to be a problem. I’ve searched Google and watched several videos on Youtube.i have yet to see this phenomenon happen to anyone else. What the heck is going on? Am I missing something very basic that I’m just not seeing. Any feedback would be much appreciated!

There are 2 aspects: Health and Endurance.
When Endurance reaches 0, a character is knocked out. They get up again at the end of combat (or if a revive spell is used later in the game). Endurance going down is represented by the portrait turning red.
When health reaches 0, they die. Health going down is represented by the portrait turning grey.

Endurance is 'healed' during a fight from the health pool - so you can also go down to dead in one fight if you keep healing enough. After a fight, endurance returns to maximum, also depleted from the health pool.

Depending on your difficulty setting, the first 'death' might be commuted to 'injury' instead.

Resting is the only way to restore health.

So in your case, the characters were knocked out.