The next Prison of Elders boss challenge for either level 32 or 34 is a servitor called Kaliks reborn.
How do I know? Grimoire.
Kaliks reborn is servitor forged by the fallen prisoners under the vestian outpost from the parts of high servitors and the ether of hopeful fallen warriors. Kaliks reborn is an attempt to bring back the long lost 'machine god' Kaliks prime which was the prime servitor for the house of wolves.
Your mission is to eliminate the reborn servitor before it can escape from the prison of elders.
Grimoire card- Kaliks reborn
From the fallen leadership section
This card isn't obtainable yet but can be found in the destiny database through many websites such as destiny tracker.
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There's also a hive boss missing..