Rex ditched attacking Voltara despite wanting to do that so badly.
Absorbing the exact same Elemental Order as the one he wanted to upgrade would give him 30% more energy than absorbing another Elemental Order. Killing and taking Voltara's would be the best-case scenario.
Voltar is a Lightning Knight, but he couldn't leave Kyran.
Kyran had done well occupying Kragos while he fought the other two, it would be too much for even Rex to deal with three simultaneously. So he wasn't going to leave Kyran here, and not to mention, absorbing Kragos' Elemental Order should suffice.
In a burst of power, Rex painted the dark sky with a colorful straight line.
He plummeted from the sky at blistering speed, faster than even Kragos's claws that were heading towards Kyran—barely a meter away. Kragos looked skyward and was greeted by Rex's claws, striking from above with an earth-shattering force.