Kill Taken Lieutenants with a fireteam to summon Champions. Seek them out at the Shattered Coast, Ishtar Cliffs, Ishtar Commons, and the Citadel on Venus.
"Hey Anzani? I'm detecting a ... you know what, never mind. I swear that frame is losing it. What? Oh, it's nothing. It keeps reporting an intermittent signal on the flagship's signal band out near Saturn. It's a comms ghost, that's all. I've done a dozen scans. There's nothing out there."