D&D 5E Tier 4+ Encounter with Asmodeus

[LordEntrails]

As my campaign nears it's end, the party is about to encounter Asmodeus. What are some good cohorts to add to support him? What about some thematic environmental hazards?

Setting is to be an oversized manor. Figured to have some fire and lightning hazards in the doorways. Maybe a few blinking portals that teleport 'victims' to other places in the manor. Encounter CR is probably 35-38. Not too worried about being too much.

For reference, party is of 6 level 20 characters with solid to high magic.

 

[Bill Zebub]

He should be surrounded by servants, innocent mortals who have been bound into servitude who Lawful or Good characters would be hesitant to hurt, but whose presence makes combat more difficult.

Of course, some of them are devils in disguise, and if they take damage they shed their disguises and attack.