Hearthpwn

Uncollectibles:

What Does This Do (and Why Would You Want It)?
Basically, this negates the Battlecry of your Demons by summoning them instead of playing them. It trades card advantage for the better board presence that a Demon with a Battlecry can bring, but without the otherwise severe drawbacks. By choosing which card to discard, the drawback of discarding is far less painful, and in particular, you can strategically get rid of cards that you think you don't need depending on the situation (ie; like a late game Zombie Chow when your opponent is low on Health, or an early game Lord Jaraxxus against an aggro deck).

The dream of course is to use it with Fist of Jaraxxus, which naturally wants to be discarded.

Which Demons Are Affected (and How)?
ANY Demon you play will consume the spell's single charge, but only the ones with Battlecry are truly 'affected' by it, since being summoned means their Battlecry doesn't trigger. For a clearer picture of how good (or not good) this is, here's a full list of such Demons and what the result would be if you played one of them after casting a Demonic Tactics:

Doomguard - You only discard 1 card (which isn't random) instead of 2.
Dread Infernal - You get a 6/6 minion for 6 Mana and nothing takes 1 damage.
Felguard - You don't lose a Mana Crystal.
Flame Imp - Your hero doesn't take 3 damage.
Lord Jaraxxus - You get a 3/15 minion for 9 Mana and nothing is replaced.
Pit Lord - Your hero doesn't take 5 damage.
Succubus - You're still discarding one card for it, but it's not random.
Void Terror - You get a 3/3 minion for 3 Mana and nothing is destroyed.

How Would the Discard Interface Work?
Tracking, only for your hand instead of your deck and "inverted" (you pick the card you want to discard, and keep the rest). Instead of 3 cards, there would be upwards of 10 cards maximum on the screen (2 rows of up to 5 each). The ordering of the cards in your hand doesn't change.

How Does This Work Technically?
This would work just like Dragon Consort, in that you wouldn't have to play a Demon on the turn you played it, and the effect would continue on indefinitely until you do. The 'play-becomes-summon' part triggers instantly when a Demon is played, and should therefore take precedence before secrets like Mirror Entity, Repentance, and Snipe, meaning they won't trigger (like when you play a Kezan Mystic).

What Happens If You Play Multiple 'Demon Tactics' Back-to-Back?
Although there is nothing to base it on, I see no reason why it would be complicated to resolve. It may simply be the case that the game could remember multiple charges for each Demonic Tactics that was cast, or that only one charge could be active at a time, meaning either you couldn't cast another until the first was used up, or that it would have no effect on any Demons beyond the first-next one you play, effectively wasting them (but still allowing you to discard cards each time you cast one, if for some reason it's a good idea to do so).

In Warcraft Mages were highly intimidated by this creatures as they devoured magic. In WoW this concept has been translated to abilities such as Spell Lock and Devour Magic. I thought to mix the feeling of both: you "lock" your opponent from casting a spell by "devouring" it and feeding on it ("Vanilla Extract" design feeling obviously hasn't work off yet).

1. As an Inspire creature - whose effect can be re-used many times - it has to be mana costed with a Hero Power in mind. In case of Felhunter, you are paying 6 mana for a 4/4 that discards an Opponent's card. Sounds extremely strong, yes, but it's the strong cards that give birth to new archetypes, and I'd be curious to see Inspire Demon Warlock.

2. Discard occurs from the deck so that the card wouldn't be completely unfair. This way your opponent will still have options in hand to deal with it immediately (e.g. removal spells). Additionally, reducing active hand size is just too strong. And finally, since this card has no practical downside, discarding from deck at least thins out your opponents cards (similar to Mad Scientist), giving them a slightly higher chance at a better draw for the card that you just destroyed.

3. Fencing Coach interaction? Yes, it's a great combo to push out Felhunter as early as possible, but if you think about it - you develop a 2/3 on T3 followed by a 4/4 on T4. You're not gaining much board control in the process and creatures can be easily removed with 1 weapon, so it doesn't seem broken at all.