Oops, forgot about this thread!
Thanks guys, I saw it too, the Ark does not cast Solar Brilliance. It casts an "Ark" effect. The "Ark" effect was not as uber as I thought it would be, it probably takes a while to whittle down the opposition. Actually that's pretty similar to Solar Brilliance too. I had to use it with Black Templars, since my only other sacreds was Ghouls, and I didn't have any sacred leaders for them. And Black Templars are not very patient

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Could be WAD that it doesn't work underwater ... some kind of shielding effect from the water.