Festus is a level 2 nurgle sorcerer who automatically start with curse of the leper and generates his other spell randomly. Contrary to popular belief not all chaos sorcerers have chaos armor, Festus runs around with a robe and nothing else but what he does bring to the table is a 4+ regeneration save and the fact that he has the mark of nurgle makes him quite difficult to hit with shooting attacks but not so much in close combat as he only has a WS of 4.
Festus Alone:
Festus isn't much of a threat by himself, fine he has the mark of nurgle to help him against ranged attacks but when anything gets close he either gets locked in combat or dies a horrible death and *poof* there goes your 185 point hero
Festus the Team Player:
Now this is where he really shines, The ability to give any unit he joins Poisoned attacks and a 5+ regeneration save really is why you would take him in a battle. Sure your pursuit move is reduced to 1D6" but you don't really want to chase your opponent when you have a block with Festus in it you would take the combat reform and face your next challenge.
There are a few set ups that Festus does great in and a few set ups that don't really use him so much.
The Following are listed by my preference
1|| a block of Nurgle Chaos warriors with additional hand weapons
Nurgle Chaos warriors are nasty on there own but when you throw in Festus, they become quite a devestating force.
Each warrior in this block gets 3 attacks at S4 which are poisonous, they reduce WS by 1 on all enemies in close combat with them, which compliments their WS 5 nicely, and ranged attacks are at -1 to hit, they also have a 5+ regenerate.
do i have to say more??
2|| next up are Khone marauders with Flails
Normaly i would never run marauders with flails unless i have a really big block of them but with Festus thrown in they instantly get a save! and when they do try and kill festus they will have to choose between the fat ugly nurgle sorcerer or a squad of marauders with flails and frenzy. Against a lot of newer players this poses a problem.
3|| Nurgle Chaos warrios with a shield
Ok, this is the way i run my warriors since they are all modeled this way but putting Festus in this group doesn't maximize the use of his abilities since, compared to block 1, they lose 1 attack for a +1 to their saves which in my opinion isn't so good i would take more attacks any day.
This block does have its advantages, it's a lot better then block 1 when you know that your opponent is going to run units with flaming attacks and doesn't lose much to its survivability when festus dies
4|| Tzeentch chaos warrios with shields
Don't even think about it, you are paying 185 for poisoned attacks and to pursue slower. Like i said in the start Don't even think about it.
Final Thoughts:
Festus is a very fun Character to run with, wether you use him to buff up a unit of warriors or marauders or you use him as a nurgle sniper that can kill low S or T value unit or just because it goes with your theme Festus is usually worth the 185 points that you put into him