I like cards with effects like this simply
because they're cute, and they imply cute plays
like making someone Flame Slash their own
creature. In practice, the effects are slow and
entirely reactive, and you can never quite
guarantee that your opponent will play something
where the impact is worth using them. Is it
really worth using Imp's Mischief to protect
your creature from Flame Slash instead of
something more aggressive like Giant Growth? Do
you get enough advantage from making them point
that Careful Consideration at you instead of
themselves to justify spending the mana?
Sometimes you will and other times you won't, so
it's a useful card to have in mind, but not the
best card to rely on every time.
Planar Chaos loved to fool around with colors
and effects, and one of the things it did was to
give black a very limited subset of blue's most
notorious parlor tricks. Imp's Mischief is a
weakened Redirect spell: you get to reassign the
spell's target if there’s exactly one target,
but it will cost you life in the process.
Despite its limitations, the card is actually
deceptively good; it's an answer in a color
you'd not expect to have it, it redirects a lot
of spells (pump spells, kill spells, targeted
discard), and it has flexible color
requirements.
The life loss is an issue, as is the reliance on
your opponent having something you want to
redirect and that you can redirect. In Modern
and potentially Legacy, though, this can pull
its weight a lot better than you'd expect, and
it's worth sideboard consideration if you're
playing a black deck.
