Does "dark prayer" ability stack? If I 3 power surges and I have 2 dark priests alive do I get 6 or only 3 skulls?

You get only 3 skulls. Dark Prayer doesn't stack. It is an attack bonus similar to the non-master Beastmen's "+1 damage" attack bonus that also doesn't stack when there are multiple Beastmen.

The -1/-1 penalty from Black Curse, however, does stack imo (even if the rulebook does not specify it), just like the Command from Master Beastmen does. A hero within 3 spaces of two Master Dark Priests would suffer -2/-2 on his attacks, -1/-1 from each Master Dark Priest.