When the Paladins are demonic and the Warlock is angelic

Our party has been investigating the Church of the Macho One (aka the Church of Randy Savage), which is our Paladin's Church, after receiving information of a demon that our paladin ticked off a long time back, had been influencing things and may be linked to the death of the previous head of the church.

After a short investigation, dispelling an illusion of the Paladin's parents, and checking the guard records, the captain of the church guard drops his cover and attacks us and the new Macho Pope. However, as the Captain screamed his head off as the lair actions triggered, I, being closest to her, noticed odd whispers around the Macho Pope.

Turns out she was the demon in question under Shapechange. The fight dragged on and we had a couple close calls, with the Paladin having to counterspell a Power Word Kill, the Rogue avoiding being plane shifted to the Abyss, and the Bard dropping a Fire Storm from her instrument to take out the guard captain.

The Demon, a powered up Marilith, was drawing power from a gem that she had captured holy souls in, which lead my Warlock to take the initiative in summoning an Arisen Nightmare (A celestial Nightmare with all it's Fire replaced with Radiant) and gave him the duty of breaking the gem. Overall he only got one hit in on he because he missed his others, but that one hit was a crit with most of the damage being Bludgeoning, which the gem was vulnerable to.

The party followed suit, including the Barbarian who was grappled in the Marilith's coils and eating a lot of damage herself, but managed one last crit on the Gem, which shattered it and caused 85d6 Radiant to both her and the Demon, ending the fight and us having to quick Revivify the Barbarian.