In my opinion, demon swarms were up until recently not very strong. [mwcard=MW1J17]Pentagram[/mwcard] is both fragile as expensive and the mana additions do not come easy during the beginning of the game. There were only 2 level 1 demons: an all-round [mwcard=MW1C15]Firebrand Imp[/mwcard] and a teleporting, piercing [mwcard=MWSTX2FFC13]Wildfire Imp[/mwcard]. In theory, the swarm could be enforced with [mwcard=MWSTX2FFC08]Infernian Scourgers[/mwcard] to make a nice level 1 and level 2 mix. All in all, nothing very impressive.
The academy warlock changes this entirely because it brings some very awesome new demons into the game:
1) Sanguine Hunter: Not much mana? no problem! This little demon only costs 3 mana! yes, it needs to take some damage, but the adramelech can use her demonic reward to heal him afterwards. 2 dice, piercing 1 and 1 armor is not bad at all!
2) Firefist brawler: This guy just punches everything untill it burns. If it rolls at least 1 critical damage, it gives a burn to its victim. For one dice, the chances are not impressive (33%), but combined with smoldering curses, [mwcard=MW1E27]Marked for Death[/mwcard], [mwcard=MWSTX2FFQ02]Bloodfire Helmet[/mwcard], Drakas, ... and many more effects he can have a ridicolously high burn chance. (80% for 4 dice)
3 Drakas, Imp overlord: Very similar to [mwcard=MWBG1C04]Slaknir, Goblin Chieftain[/mwcard], this guy will boost all demons in his zone.
4) Mheggeden, Sealed demon: One of the big difficulties with swarms is the huge set-up time required. Mheggeden allows you to summon an amazing demon at the start of the game while still not mana draining you. Of course, you have to pay some way, but at least your swarm will have a big guy supporting them from the beginning. In general, I like these kinds of new cards a lot. (Ehren, Slavorg, and lightning raptor in lesser extend)
These are the main 4 cards that really buff demon swarms a LOT. I left out the combustion demon, because I don't think he is worth it in arena. He's solid in academy though. There are also some very solid level 2 demons outclassing the scourger many times (Afflicted demon especially). Sharkbaits aggressive fireball-adramelech warlock has been rocking for a long time, but I believe demon swarms might actually become very viable now.