Xillox is a boss enemy that you can fight during a Wrathborn hunt in Season of the Hunt. You will need a fully charged Crypolith Lure that you can get from The Crow on the Tangled Shore. You will first need to charge your lure by playing Crucible, Strikes, or Gambit, then right-click on the Crypolith lure in your Quests tab.
Place the Xillox Prey Mod in the Prey Mod slot, and then place whatever additional mods you have access to and want to use in the other slots. If you have none, just place empty mods in there.
Now, check your map on the Dreaming and you will see a new icon of the location of a Crypolith that will start the Hunt. Go there and walk up to the Cryptolith and activate the Lure.
A Hive Ogre will appear, and when you lower his health enough he will escape. There will be a small Hive ball of energy left behind, and you can interact with it to track him. Follow the trail of green energy on the ground to another podium, potentially in a different region, and interact with it again to be whisked off to the boss fight.
You should end up in the Steppes, and will need to follow the trail of green energy to the next Cryptolith. Activate it and the Ogre will return. Finish him off and then prepare for the bossfight. Xillox, a powerful Hive Wizard will spawn, and will be accompanied by some Knights. Take out the Knights to form a green pool on the ground, then jump in it to get the Wrathful buff. Now you can take out the Wizard’s shield and kill it.