That's a C'tan shard of Mephet'ran the Deceiver. The cult is building it an avatar or a host body. Probably another two reside on the main battle barge (or are part of the Void Dragon, who knows?) and are chained by the inquisition. They refer to the new orb as "brother". Probably the contact between the two is made by the devices installed by the kill team. The Inquisitor overheard the conversation "the astartes defy our touch", and after "we must take the offer" the Inquistor screams over the vox com link "Return them immediately". The Captain gives order to extract, then the Inquisitor breaks under the strain as he is distracted and starts to get possessed. Anti possession protocols are engaged for an awesome scene. The Sergeant gets the orders to fall back, but it's too late and gets sucked into the shard. Rolls one on the plasma shot. The battle barge proceeds to apply Exterminatus, killing the shard and teleporting our kill team squad on what appears to be the Necrontyr homeworld, probably the royal family tomb. You can see little flashes on the other "platforms" as the Sergeant overlooks the nightmarish vista as his other teammates teleport in.