Just a note - there will be spoilers to Path of the Renegade, but I will spoiler-tag them out.
I was just trying to piece together Arhra and what he is.... so what do we know?
- He's the Father of Scorpions. A Phoenix Lord.
- He created the first Incubi Shrine.
Not much really... so what is implied?
- He's Drazhar. (Implied very very strongly)
- He fell to Chaos (although this could be "old fluff" and may have been semi-retconned with the new DE Codex. The CWE Codex said Arhra "Burned with the light of Chaos" but then again that could just be perception...)
So... Drazhar's art shows him with a cracked (unusable) Soulstone. So Arhra cannot be "put into it" ala the other Phoenix Lords.
This points to the fact that Arhra must be afflicted with the Soul-thirst that all the other Dark Eldar have.
So he's been killing his way to survival for the last god-knows how many years... but is this possible? Does Arhra still exist inside Drazhar's suit of armour?
Onto the second point.... and the spoilers for Path of the Renegade.
So what I'm trying to work out is...
What IS he?
WHO is he?
How has he survived so long?