Malok, Pride of Oryx has appeared before you. It is time to defeat this Taken before his power can be cemented and the Taken become a threat once again.
Malok will periodically change his tactics. This is your marker to know what is going on. He will alternate between trying to blast the Guardians and summoning in a number of Blights with supporting Taken.
The Blights pose a major problem because of the Shadow Touched status they inflict. This slows you down and stops all jumping abilities.
A useful trick is to head into the small room in the back of the circular shrine area. A team can easily fight off the incoming Taken and just as quickly destroy the Blight that appears here. This can be done in most areas of the arena, it just means that someone will need to check for the Blight periodically when you see Malok is changing tactics.