Lord Aelil was one of the original nine Sidhe, and the founder of House Aelil. He was a prime mover in many of the events that led up to the Shattering.
Like most of the House Founders he was present at the Battle of the Riven Mountain, though it seems that he was on the side of the Trolls. He was discovered at the end of the battle in the Halls of the Mountain King, disappearing before he could be captured by Lord Gwydion and his followers.
[I think the following events should be taken out into their own entries.]
Lord Aelil returned to Arcadia, prior to the Sundering, and summoned all the Sidhe to him. When all were assembled, Lord Aelil tells them of the coming strife, and lays the blame for it squarely at the feet of those who defeated the Trolls at the Battle of the Riven Mountain. He had made this point before, at the Gathering of the Sidhe Host, and now he wanted to do something else to try to avert the disaster. What was his plan? Was this all just a trick to get Lord Dougal to break the Gate, etc? Lord Aelil was certainly known for his foresight....
The other Sidhe are upset at this, and Lord Gwydion says that, on the contrary, it is the traitors, those who did not fight with the Sidhe, who were the cause of the coming problems, and that of those traitors Lord Aelil was the worst, for not only did he not help the Sidhe, but he also helped the Trolls. Lord Aelil retorted that they were fools if they did not see the truth, and turned his back, prophesying doom as he walked towards the Gate of Horn.
Lord Dougal went after him, then, and struck at him with his hammer. But Lord Aelil seemed to know, unseen, what was happening, and he avoided the blow, which landed instead on the gate, and so mighty was the blow that a piece of the gate splintered off. Lord Aelil immediately grabbed this shard and with it pierced Lord Dougal. Whereupon Lord Dougal's soul was captured in the shard and his form became frozen as like a statue, the shard still buried deep within it.
After struggling for a few moments to free the shard, Lord Aelil turned once more and made to go through the gate. As he did so, Lady Eiluned grabbed him around the wrist. Lord Aelil gave a great scream and disappeared. Lady Eiluned then left without a word and was not seen or heard of again, save in the stories of Pooka and, occasionally, Eshu.
Lord Aelil returned a second time to Arcadia, after the Sundering. He stole the statue of Lord Dougal and brought it, complete with the shard of the Gate of Horn to Nocker. Nocker removed the shard from the solid form of Lord Dougal and from it forged the sword Slahan.
Armed with Slahan, Lord Aelil ventured out into the Dreaming, seeking Sidabras. When the two met they fought, and after a great contest which broke mountains Lord Aelil won, with the aid of the magic that coiled within his belly. Using Sidabras' bones to aid him, Lord Aelil tried to create a new Arcadia, but succeeded only in blasting utterly the region he was in and bringing about the Shattering. His shade wanders there still.