Chipping in here: I don't agree that Devour in general needs this limitation putting on its mechanic.
If a spell with Devour is too strong and the troop is everywhere, it's the troop / spell that should be fixed, not the mechanic.
I think moving Kerberos to a 50% chance was a mistake - but even that on its own, at 16 mana cost is probably okay.
I think Forest Guardian was a wonderful troop idea and an interesting third trait.
But put the two together? Suddenly (with an easy +2 purple set-up), one purple surge fills the Kerberos and it's a coin toss to turn the game. Yeah, blah, it can be drained, countered etc. But not every time.
Most modest solution is probably to take the Beast troop type off Kerberos.
Thematically I don't like the Forest Guardian (an avatar of nature) giving mana to a burning daemon (not very natural, even if it looks a bit like a dog).