The terms of the treaty which Kion arranged were as follows: The dragons were returned their capital and its surrounding territory. In return for this the dragons agreed to marshall their forces and go to war with Aerykun in the North.

In the North, when men still ruled, there had been three great fortress cities. These were renamed when Aerykun conquered them, being given the names of Darkness, Gloom, and Doom. The fortress Gloom was the furthest south.

The dragons and their war aside for a moment, we come to one of the most critical moments in Red World history. In the Shadow Realm, where neither man nor dragon watched, Aerykun had made in secret an object of great power. [In the Red World, there is a cap to the amount of power which any being may wield at once. To bypass this, Aerykun poured great power into this object]. This "object" was made in the shape of a cross, and to the eye it appeared either as burning wood of black hue or as onyx stone.
With this as his banner, Aerykun had intended to extend his control from the far north of the Red World to that which remained to be conquered.

The dragons and vaeances began their long flight from their lands to the north. It is said that there column was over a mile wide, and many miles long, dragons and vaeance flying wing-tip to wing-tip and tail to head. The farmers below appreciated the shade for hours, and so long as they were not struck, the droppings as well.

The dragons struck, this time coordinated with the vaeances, the Fort Gloom. They razed it utterly and slew its garrison. Kion returned to Okthin to give tidings to the king of men. His report was thus, "Your enemies are slain. Your friends are free. As for us, fourteen lie dead, for the Iron Beast was with us again." Iorin had come to their aid unexpectedly, and thus they had only fourteen casualties.

But where was Aerykun as his castle fell? The moment his spies reported of the bargain between men and dragons, he had departed the Shadow Realm and went straight way to the castle Gloom. He had waited for scant days when a second report came: the Black Cross had betrayed him. The power with which he had endowed it was so great that it had brought itself to life, and had usurped his throne. He flew from the Upper Red World back below to try to regain his kingdom.

But fate was against him. Several of the demon lords which he had set up would join him; the rest, in the majority sided with the usurper. Despite their great power as individuals, and the demon lords' armies, they could not prevail. When they could not regain Aerykun's former palace, Aerykun and the two demons fled. The mightiest of the demon lords fled alone to the mountains surrounding the Shadow Realm, which overlooked the Fires of Passage. Even as the Black Cross's minions surrounded him to capture him, he leapt into the Fires.

The others who had sided with Aerykun were captured and, after punishment, reinstated to the Black Cross's service. Aerykun himself escaped to Gor Xander Burld, a world of unknown relation to the others, and into which the Black Cross locked him that he should never escape.

They have traitors, too.

Ooh...this is my favorite part so far! I like the idea of the Black Cross as opposed to the White Cross earlier. Wait, where is the White Cross in all this?

Geographically, thanks to the recent reforming of the Red World map, the White Cross is in his fortress up at about the northern pole of the big globe.
Now a little more toward what you meant in your question, he is silently orchestrating events through his underlings. Much like the "Will of the West" in LOTR, his work is silent, subtle, and concealed. His underlings, at this point, consist mostly of the dragons, because the men have not learned magic enough to work with him directly. Man-wizards will eventually be the representatives for him on the Red World, guiding the rest of mankind.
Ever since the worlds split it has appeared that the White Cross is sitting back doing nothing. While he himself is not wielding his power, he is watching and working through others.

So was he for the dragons or the men in their wars? It doesn't seem like he would sit back and watch without orchestrating events while all this bloodshed is going on in his world.