The way challenging works is a ready (untapped) character exerts and chooses an opposing exerted character to challenge, and then both cards compare their strength (attack) and willpower (defense) values to each other (meaning the attacker automatically gets hit back by the defender)
Even more so when you consider the limitations on shipping that the Secret Lair line has
I do wish that it werent sold at a premium over Standard sets, but who knows what Standard sets are going to cost when they bring Universes Beyond into the mix
Do they look exactly like real Magic cards
You can ramp hard, have a low curve or go straight down the middle
Similarly, Aesi, Tyrant of Gyre Strait can be a great start for a Tatyova, Benthic Druid build, while still keeping the original commander in the deck