Yanno, my first reaction about a nic/Mavra team up was meh. But I could see Nic thinking the only real hope for reality is banding the powers of "dark" together to fight the incursion from outside. After all, the gates are protected by Mab.
If he was working from that line of thinking, it would more or less explain all of his actions. He has had 2000 years to snatch up power, but he doesn't seem to go in for controlling or toppling governments, he could probably own most of a continent by now if he wanted to. But in the last ten years he has made a play for power from the plague curse, the archive, and the artifacts in the vault. Certainly sounds like he is accelerating his grab for power.
He knows a war is coming and it even sounds like we know what side he is on. It would be precisely within his character to think that he is realities only hope, and uniting other powers behind him (dark ones obviously) is how he can do it. And if he happens to come out of it with enough power to make himself god-king of the universe in the aftermath,
So much the better.