The over-numbered 136/135 print slot marks it as the secret card beyond the set's main 135
You may cast spells from among those cards for as long as they remain exiled, and mana of any type can be spent to cast them
Originally, when a card with madness was discarded you first had to choose whether to put it into exile or to let it go straight to the graveyard
There is some amount of variance to how wed deploy the Dragon Storm Chain depending upon how much mana (and which combo pieces) we have access to on the turn we Storm off but generally if we have ample mana to go off the ideal chain is: TIAMAT MOROPHON MIIRYM TERROR OF THE PEAKS (we get a copy of Terror of the Peaks so each subsequent Dragon we cast will get two sets of ETB Damage triggers)
In EDH, where life totals start at 40, it has an alternate job of keeping life totals low, and turning this into a 0/40 has its combo potential
For many of the same reasons that Jace, Architect of Thought is better than FoF these days these Charms are better than the hypothetical instant speed Council of the Soratami