Archetype: Tempo Control Highlander
Skill Cap: Above Average
This deck originally started out as a fun deck that took advantage of effects that benefit from dealing damage to your own minions or your hero, hence the name Masolock ("Maso" from Masochist).
Somehow, every expansion after it kept on releasing cards that fit perfectly in this deck. Suddenly, it was no longer a joke deck, and is actually now semi-viable.
Defile: Very strong board clear on its own. More importantly, it can be played together with Grim Patron to clear the board and give you a board full of Grim Patrons. Defile has a hard limit of 14 casts (it will not go infinite with Grim Patrons). Go grab a coffee while the animation plays out!
Reno Jackson: This card is what allows you to continuously punch yourself in the face without dying.
Krul the Unshackled: Previously, I would run into many games where I could no longer afford to play most of the Demons in my hand without putting myself at the risk of dying. Krul the Unshackled solves this problem, and coincidentally this deck already runs no duplicates.
Bloodreaver Gul'dan: Since this deck already runs so many Demons, this is a no-brainer. The Hero Power, Siphon Life is extremely overpowered on its own and also helps us to counteract the self-damage.
Gurubashi Berserker: Can get out of control really quickly if the opponent doesn't have an immediate answer for it.
Floating Watcher: Can get out of control really quickly if the opponent doesn't have an immediate answer for it.
Elven Archer: I know what you're thinking. Elven Archer? Why would you play this card? This card is extremely flexible. It can be used to target your hero or your own minions for self-damage effects, ping 1hp enemy minions, and is a great activator for Defile.
Blingtron 3000: Mostly included for fun. Attacking into a minion buffs Floating Watcher. It also doubles a semi-weapon removal (chances are, it won't give them something better than Gorehowl/Atiesh/Frostmourne/Shadowmourne/Blood Fury/Ashbringer/Grave Vengeance). Sometimes you can actually aggro down the opponent with the help of this card. It also has a chance to outright win games if it gives the opponent Cursed Blade.
Unstable Ghoul: Taunt + AoE. Also damages your own minions.
Voidcaller: This card is even better after the Ironbeak Owl nerf, since not many people run Silence anymore. Can outright win you the game if it cheats out a Pit Lord/Dread Infernal/Abyssal Enforcer/Krul the Unshackled early on. Alternately, it can force the opponent to waste a Hex/Polymorph/Entomb, which is still really great for its cost. The downside is that this card may pull out your Krul the Unshackled when you don't want it to (eg. when against Control decks).
Bane of Doom: This card is still preeetty good in Wild, with more good Demons than bad, some of which can outright win you the game. Edit (Gadgetzan): This card became way better with the addition of Street Trickster, Abyssal Enforcer, Krul the Unshackled. Edit (Un'Goro): This card became slightly better with the addition of Lakkari Felhound. Edit (Frozen Throne): Great synergy with Bloodreaver Gul'dan. The downside is that getting Unlicensed Apothecary can also screw up your Bloodreaver Gul'dan. This card is also now slightly worse with the addition of Howlfiend. One of the top candidates to be replaced.
Kabal Trafficker: Has potential to give you more cards that have synergy with your deck. Great synergy with Krul the Unshackled/Bloodreaver Gul'dan. The downside is that getting Unlicensed Apothecary can also screw up your Krul the Unshackled. One of the top candidates to be replaced.
Dark Peddler: A great 2 drop that can Discover more activators for your deck. Best Discovers are: Flame Imp, Mortal Coil, Voidwalker(helps you stay alive longer, or protect high priority targets), Power Overwhelming, Soulfire, Elven Archer, Tentacle of N'Zoth, Sir Finley Mrrgglton(when your hp drops too low to Life Tap anymore), Shifter Zerus, Voodoo Doctor, Young Priestess, Zealous Initiate, Mistress of Mixtures, Emerald Reaver.
Kazakus: Since we have no duplicates in the deck, this card is a no-brainer. A 1 mana Felbloom is a great activator, Goldthorn allows your minions to be damaged/activated further, Shadow Oil is insane with Krul the Unshackled, Stonescale Oil helps to counteract the self-damage.
The Lich King: A very powerful card on its own. Most of the generated cards have great synergy with this deck. Death Coil's heal can be used to negate self-damage. Obliterate/Frostmourne buffs Floating Watcher. Anti-Magic Shell is extremely powerful with Grim Patrons/Demons on board. Army of the Dead can be used to pull out Demons from your deck for Bloodreaver Gul'dan synergy. Death and Decay is a powerful board clear. Death Grip can potentially ruin opponent's combo.
Drain Soul: Decent removal + healing.
Kabal Courier: Potential to discover win conditions.
Fencing Coach: Allows you to tempo out Floating Watcher+Life Tap on a future turn for 5 mp. Can also be used to smooth out your curve (when you have 3 mp leftover and have nothing else to play). Removed from the deck because of the lack of synergy, and the card is pretty weak on its own.
Violet Illusionist: Allows you to play your self-damaging cards without damaging yourself.
Blowgill Sniper: Can be used as an activator.
Bomb Squad: Has potential to turn the drawback into a benefit, however rarely will it not die during the enemy's turn.
Ancient Shade: This card used to be included for the self-damage theme and for the moments when you draw the Ancient Curse with Floating Watcher in play. Can sometimes win games against Control decks if played on curve and they don't draw answers. Also a great target for Shadowflame.