At a cost of 17 mana, Cerberus is the 2nd most expensive Demon to date, only behind the Lord of Fire himself. He has a good amount of armor and health, 3 and 13 respectively.
His attack is not too powerful. Quick attack of 3 dice with a +1 Piercing. Not too great right? Well, that changes when he is guarding.
As you can see from his text, when he is guarding in a zone with a Dark conjuration you control, he gains Unmovable and his attacks gain the Triplestrike trait. So basically when he's guarding a Dark conjuration, he cannot be pushed out of the way. & when he gets attacked, each of his heads makes its own counterstrike. This is very thematic considering that he has 3 heads and that the Cerberus of mythology guards the entrance to the Underworld.
My potential issue with it is this: how often do you need a Dark conjuration that needs guarding? Most Warlock and Necromancer builds are Aggro, so they'll want all of their creatures fighting on the front lines, not hanging back protecting conjurations.
Now the exception to this would be the Altar of Skulls for the Necromancer. This is a GREAT guard option for it. The only issue there is that he doesn't have Intercept so he can't guard against Ranged attacks.