LotD can't harm the Avatar

I step forward and see shapes moving in the flames, large and bulky. Space Marines. Survivors? I call out.
‘Well met, brothers…’
I trail off as the figures step from the flames. They are Space Marines, but they are not my brothers. Hulking black shadows wreathed in witchfire, they are of sinister aspect, ebon armour decorated with arcane sigils made of what looks like bone. They are skull-helmed like Chaplains, and their bolters lift as they stride relentlessly forward.
As I see them, I feel a curious sensation. I am a Space Marine, and I know no fear. But as I look upon these apparitions, their armour cracked and broken with cold, pale flesh and bare bone showing between sundered plates, a shiver passes through me. As I gaze upon their infernal countenance, I realise that while the daemon behind me is a threat, these abominable Space Marines – so like me, yet so very different – are more dangerous by an order of magnitude.
Fingers squeeze triggers and I prepare to feel their rounds strike my battleplate, not knowing why this is happening, angry and confused. But the shots pass me, trailing flames. As adrenaline surges – I am still alive – I turn and see the daemon. It raises its spear and casts it towards us. I throw myself to the side, but the spectral Space Marines simply continue to walk. The spear passes through them as though they were mist, before arcing round and returning to its wielder’s burning claw.
Utterly silent, the dark warriors continue to fire. The creature roars and sweeps its spear around in a great arc, to no avail. The Space Marines loose more rounds, the flaming fury of their weapons equally ineffectual against this god of fire. Fire.
‘The fire!’ I exclaim. I open my vox to every channel, hoping that the silent figures will hear. ‘Fire,’ I say. ‘It is a creature of molten fury. Your flames cannot harm it.’
They hear. As if guided by a single will, they pivot and loose more shells, this time towards the glass dome. A barrage hits it and cracks spread across the surface. For a moment, the universe seems to hold its breath, then breath becomes irrelevant as the glass breaks and the air rushes from the chamber, taking bodies, eldar and Invaders alike, and other debris with it. In an instant, the fire is snuffed out and the spectral figures vanish. They came from the flames and now leave with them.

From the Flames