Horobi, Death's Wail
 
The symmetry on Horobi's ability will probably be a turn-off for some players; to be fair, though, it's hard to make an effect like this asymmetrical while still giving it a cost that anybody can actually pay. The fact that it's a double-edged sword and allows, for example, direct damage spells to kill creatures that would usually have too high a toughness makes it more appealing for theme-based decks that specifically build to take advantage of it. You could go the Privileged Position route, or kill huge things with Prodigal Pyromancer, or even just stuff your deck with things that look for creatures dying and ride the chaos. The important thing is, Horobi poses lots of questions for the deckbuilding process, and that's a good thing. 
 
Constructed: 2/5
Casual: 4/5
Limited: 3/5
Multiplayer: 3/5
EDH/Commander: 4/5

Horobi, Death's Wail (2/6)

Horobi’s stats are pretty acceptable for his spot on the mana curve; a 4/4 with flying for 4 mana is actually a really good deal. What makes Horobi unique isn’t his stat line, though; it’s his ability, which attempts to kill anything that gets targeted by anything.

This changes how a lot of things interact, and the list includes:

• making Equipment useless. Since Equipment attaches to target creature as its ability, that creature dies.
• turning burn spells into Murder.
• giving cheap, reusable targeting effects lethal power. This includes things like Hex Parasite’s ability to remove counters, since it activates for 2 life.
• turns your targeting lands into lethal lands, like Tower of the Magistrate and Urborg.
• gives a use to weird and useless old cards like Touch of Darkness.

Horobi makes the game very weird. Unfortunately, he's a fragile creature, on virtue of his own ability marking himself for destruction when he gets targeted. He's a blast in Commander, since he forces the game along a different axis, but I think you should stay away in Constructed.

Constructed: 2

Casual: 5

Limited: 4

Multiplayer: 4.5

Commander: 5