Analyzing log of fight with Silent Sun during Brass Palace, I see no such emote: "sprouts a cloak of living lava"
I do see:
[Fri Jan 17 05:37:29 2020] Silent Sun begins casting Cape of Lava.
And that's when these messages start:
[Fri Jan 17 05:37:29 2020] YOU are pierced by Silent Sun's thorns for 9732 points of non-melee damage!