|Who are the other two people in the picture?|
As a defender, you'd better hope your front realm has some sort of movement restriction. "Can only be attacked by flyers or swimmers" is good. "Cannot be attacked by clerics" is better! Because if Istus can get to your formation, you're in a heap of trouble.
Before combat, Istus allows her owner to grab two cards from your hand - and it's not at random. No, the opponent gets to take a good long look first, then pick your two best cards and send them to the discard pile.
In Standard, this is a brutally effective power. In TAV it's just unfair. There goes your two best instant-kills. Or both your counterspells. Or whatever Istus's owner wants you to be without.
Of course, you could avoid losing cards by letting the realm be raised without combat. Then you get to deal with the same problem next turn. Maybe you've got a Pit Trap? :)
It should be mentioned that Istus can also be used on defense, although that's not as much fun (and no sane Spellfire player is going to attack her anyway, unless they have a way around losing their two best cards).
Even if you've got a fat enough hand to beat Istus, the damage has been done.
Probably the most dangerous Avatar ever printed, this Lady is a beast no matter what version of Spellfire you're playing.
Next time: make a Wish!