Well, it's not exactly easy. I had no astral, no death and no water income - bad luck with gem sites. With just 2 death gems per turn from my pretender I couldnt really afford a big army of Umbrals. Not until Bandar Log sacreds, summons and massive communion would make a handful of Umbrals obsolete. So I rushed and failed.
Oh, and my luck dominion yielded me some water gems twice and generated two 'master thief stole your magic gems' events. So you can do the math, how long would it take me to summon a whole army of Umbrals.