The command ability is still to be revealed.
The trapped Chaos god was a major protagonist in the lore accompanying the Daughters of Khaine release earlier in the year.
Ultimately your goal with Keepers is pretty straight forward, they are powerful in melee, outfitted with many attacks and a decent save.
Clocking in at 12 T5 Wounds for only 80 points the same cost as a Hellflayer with 6 T5 Wounds , the Exalted Seeker Chariot actually functions excellently to soak overwatch for your more valuable units or screening out wherever is needed.
They orchestrated Horus's corruption and the entire Horus Heresy.
As children of Slaanesh's degenerate dreams, Daemonettes bring all he has to offer to the fore.