OK, we know what Icarium is. Calm spells it out in TCG:
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The two would seek each other out – they could do naught else. The dragon is negation. But Icarium is an open wound into Chaos itself. When his self shatters, when his so-called rage is unleashed, he is but a conduit, a portalway. This is why he cannot be stopped – he is not even there. Shall you do battle against chaos itself? Impossible.
We see this process begin when he's fighting everybody at the T'lan Imass Throne in BH.
But we have this, from Phyrlis in HoC:
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'Foolish, unmindful T'lan Imass, to drive a spear into the grounds of an Azath House.'
'What did they know of houses, Cynnigig? Creatures of caves and hide tents. Besides, it was already dying and had been for years. Fatally wounded. Oh, Icarium was on his knees by the time he finally delivered the mortal blow, raving with madness.'
And Cotillion says this in BH:
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"It is less a curse," he finally said, "than a … residue. The death of an Azath House releases all manner of forces, energies -- not just those belonging to the denizens in their earthen tombs. There is, burned into Icarium's soul, something like an infection, or, perhaps, a parasite. Its nature is chaos, and the effect is one of discontinuity. It defies progression, of thought, of spirit, of life itself. Mappo, that infection must be expunged, if you would save Icarium."
Icarium does not seem to be a portal to Chaos when he's on his knees, raving after having dealt the Azath its death blow. But he was infected with chaos at that moment! He was hugely powerful when he went to free his father (he thought he was freeing Gothos), and
became the portal to Chaos as a result.
Yeah?
'All Eres were bonecasters, Trull Sengar. For they were the first to carry the spark of awareness, the first so gifted by the spirits.'