Using other cards to add DON
All you have to do is have Jumbo Cactuar out and able to attack, Jarad out, and three mana open during the attack step
You can play any card in the game, but your mana base needs serious work
Using played/damaged cards is perfectly acceptable in a lot of groups, especially for rarer cards that are now twenty years old and exceedingly difficult to find in near mint condition
Also, they love it because you can show up with a couple of dice and a piece of paper and have all you need to play in a real Magic event
But from what Rosewater wrote on his blog, impending is a 6 in the Storm Scale, an informal scale which goes from 1 to 10 and which Rosewater uses to communicate how likely it is for a mechanic to return in a future set (1 being extremely likely, 10 being very little chance)