Mechanics that reduce or eliminate costs are only really usable with either low-cost cards (like Secrets in the case of Mysterious Challenger and Cloaked Huntress) or in small fixed amounts (reduce the next spell by 3 like Preparation, reduce your next dragon by 2 for Dragon Consort). It’s the only way to prevent a player from getting way too much tempo out of a single card combo.
 
The exception to this is in cards like Deathwing: Dragonlord, where its ability can be countered before it goes off and you have to actually end your turn and wait for it to occur. Delayed effects are weaker than immediate ones.

Ridiculous Malygos combos go!

@Lewd  
Pray that it doesn’t get stolen by Priest or polymorphed by Mages and Shaman.

Yeah this makes the infamous Dr.7 look fair in comparison. I’d run it with any deck with high costing dragons, two big bodies on turn seven with many dragons having neato effects. Early Ysera or Deathwing would be particularly nasty.

1.Summon Netherspite Historian. If you get Deathwing Dragonlord, grab it. Make sure you have multiple Dragons in your hands. Legendary makes it better.  
2. Summon Ember.  
3. Summon Deathwing Dragonlord.  
4. GG.