You nailed it. He
He will kill the rest. The one left alive (if any) will be his mate. Some males are just too aggressive to live with even females of their own species. There are probably some mellow exceptions to the rule too but I sure wouldn't bet on having one.
It's a harsh process. Umbees are ruthless. In the wild they can flee but in an aquarium there's nowhere to run. You may be able to rescue some of the unchosen before he kills them but you'd have to be very quick to remove them the second you saw him beating them up. More than likely you'll just find them floating.
The lucky lady (if any) will be allowed to live but she'll need a ton of hiding places that he can't get into, pick up and move, overturn or destroy. A divider with a few holes big enough for her but not for him is a great idea too. If he wants to breed but she's not ready to lay eggs she will need that protection. He may be nice to her most of the time but things can go sideways fast.
It pretty much sucks to be in a tank with a male Umbee. Unless, of course, you ARE the male Umbee. Then it's good to be king.
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That helps a lot. Thanks