Well i was leading in hard mode with the Morgarw, them came the faction quest step from hell !!! a insane overpowerd roving clan army came and absolutly destroy me, absolutly nothing could kill them. My heroe died 3 times !

I had that happen to me as well. It seems like the quest can get to that stage (5, part 2 if I recall correctly) very quickly and then have this stupid army that can wipe the floor in an aoe thanks to the circular attack and sweep strike back. 

Lost all my cities. 

i´m glad it is not just me them  

Pursuing the quest fast should reward you, not punish you. I started another game in hard mode, but i stoped pursuing the quest when he tells me to search all ruins in a ocean.   

I ran into the same problem when I was playing a game and trying to get the achievement where I don't build or buy any land units.  I'm not sure how I'm going to beat that army yet.  They one shotted my fully upgraded masterminds and 2 shotted my fully upgraded and equipped heros.

They spawned in trasport ships on water for me, so were easy pickings. Maybe they are overpowered because you are supposed to fight them on sea? Might just be era/quest pacing problem and I lucked out.

No, they came from land and sieged my capital. It is about timing, because i did the quest too fast, and lost the game. 

sounds tragic