Worst Fears was playable in Standard. Is it good in other formats like EDH?
Controlling your opponent's turn is always fun. Yes, Worst Fears does cost eight mana - seven colorless and one Black - which is a bit pricey. But, considering that casting Mindslaver costs 6 mana to cast and the activated ability costs 4 mana, this is a relative bargain. Unlike Mindslaver, though, you can't reuse this card, as it's exiled as soon as it's played. As an artifact, there are many ways to get Mindslaver back into play and use it again. That reusability is why the original Mindslaver is so good, after all. Being a one-shot effect, is this really worth playing?
Still, Worst Fears can be a pretty scary card. Especially late in the game, being able to control an opponent's turn can be devastating. Also, having only a single black mana in its mana cost makes it playable in a wide-array of control shells.
Even then, many players consider Worst Fears a waste of a mythic rare slot. Then again, I can understand them not wanting to print an effect like this at rare for Limited purposes - even if it was on Mindslaver back in the original Mirrodin days. In today’s Magic, being able to draft this at rare would have made it too much of a problem. That’s why when Mindslaver was reprinted in Scars of Mirrodin, it was bumped up to mythic rare. Just as it was in its original Standard heyday, it was yet again an important card in that Standard format, as well.
Modern Tron made heavy use of Mindslaver to great effect for many years, so it was thought Worst Fears could be played a bit in Constructed. Of course, eight mana is a lot, regardless of the format it’s being played in. But, the 8-mana sorcery did sneak into a few Standard decks back in 2014. It did see play in a few Mono-Black Devotion decks, a major competitor during Theros block Standard. One 8th place Standard PPTQ UB Control deck ran a copy in the sideboard.
Pro Ali Aintrazi even played a mainboard copy of Worst Fears in his January 2015 Sultai Delve deck at a Standard Open in Washington DC. A "Mono Green" deck even ran one copy in its sideboard, as this Green deck splashed Black mana for Pharika, God of Affliction and Doomwake Giant. Since early 2015, it hasn't seen much in the way of competitive play, in Modern, or otherwise.
Worst Fears does see some play in Commander. The Commander who's made the best use of this sorcery is Jeleva, Nephalia's Scourge. Jeleva has an ability to exile cards from the top of your deck, and you can cast any instants or sorceries from those exiled cards without paying their mana cost. This means you can play Worst Fears for free whenever you want. In other Commander decks, it's just not really worth an 8 drop slot.
The later in the game Worst Fears is played the better, so it's a good effect. But, since you only get one shot at it, you had better play it at the perfect time. Really, it's best if you can find a way to cast it for free. Otherwise, it's not worth casting in most decks. While it had its uses back in its Standard legal days, nowadays you'd be better off with Mindslaver.