It was intented to be achieved via three great prophets+the sphinx. If the difficulty is to be preserved the third goal should become "control the sphinx and establish three great prophets to your capital" (so that Jerusalem doesn't count).
Mongols:
The UP could be:
Power of distruction: No stability penatly from razing cities.
While their UU should have this attribute:
Keshik:
*3

*12:str:
*Keshik stacks spread terror to weaker cities
*50% withdraw chance (historically speaking, that was the main power of keshiks, they withdrew battles, weakened their enemy until they could conquer it)
(No caulateral damage, they are just stronger)
Terror should be connected to keshiks and not unique power, else it can be exploited in later eras.