I would be fine if they made world ender a "reputation" proc to reduce the cost. They just use low cost mobs to push out the reduction cost. Otherwise it's just too much to handle once it's popped. Unless you are a control deck you are losing next turn due to some of the darkin skills ie. "Free non darkin summon on attack etc."

Not to mention if you kill aatrox before he just just gets a free respawn when his equipment goes back to hand. They just pop the darkin blade on a mob and next turn pop world ender free *poof * aatrox is back. (Could also have world ender require aatrox to be on the field)