Necron Monolith with 2 squads of Necron Warriors that will be replaced when destroyed will spawn at the middle relic once any unit (including infiltrated Gretchin) reaches it. Monoliths have 8k health on insane (same as in imperial mission) and it will stay there. Gorgutz will be teleported there and you will get Shoota and Sluga Boys squad on the site too. Crull will escape northwest. Exploration of map is reset at this point. Not long after first monolith a second one with the same support will spawn northeast near a crashed ork plane. It will stop to destroy titan weapon while Necron Warriors will march to your base. Third monolith will land right next to southern imperial outpost at the foot of the titan and it will deal with imperials first.
When monoliths are destroyed, Crull will spawn behind Chaos base and all you need to do is get Gorgutz near him for mission to end. Expect full chaos arsenal on the way there except Possessed Marines and Defilers.
There are 2 crashed ork planes, one northeast of titan that does nothing and one near the western corner where you will get Killa Kan, Shoota Boys squad and ability to create Looted Tanks.