The main thing to look at is piety & badboy.
Badboy(known as BB) is a nasty thing, as it does not appear directly on any screen. You've got an inderect value when hovering your mouse pointer over your vassal's portrait : you'll get -0,4/100 loyalty per month per BB point.
You earn 1 badboy point per assassination attempt(succesful or not), per declaration of war, per county conquered outside crusade, and per title claimed(bought by prestige). Badboy is extremely nasty as it destroys your relations with others, & destroys your vassals loyalty. If one vassal gets disloyal, he breaks, you beat him, which grants you 1 BB point for conquering its county, then other's loyalty gets even lower, then other get disloyal, then......
BB goes down only when you die & your heir inherit(-33/100) and when your piety is positive. Count 1BB less per year around 1000 piety(exact formula unknown, it seems logarithmic). So piety is faaaaaaaaaar more important than prestige, as if you don't have prestige, you're still alive. If your piety is under zero, your fall is nearing.
Thus the interest of the beta patches(though they say 1.05 is not for long) as they provide with "targetted" crusades, which are extremely juicy in terms of piety & prestige(2000 piety & 1000 prestige for the target itself, & already much for provinces around).
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