Whenever I see a card that specifically
references cards other than itself, I think of
the Odyssey block. That was a theme it touched
on, in cards like Pardic Firecat and Diligent
Farmhand, but Foul Emissary is much broader in
the specific cards it wants to work with. This
is good - while your options are still limited
by the fact that emerge has only been in one
expansion, you can feel free to pair it with any
emerge card in any color or strategy. Plus, the
fact that its comes-into-play ability is a
toned-down version of Lead the Stampede means it
has a chance of sneaking into some constructed
decks in smaller formats like Standard.
Foul Emissary is clearly meant to help curve
into Emerge; it hilariously curves perfectly
into Distended Mindbender (from Wednesday), and
it even leaves behind an additional 3/2 body if
you use it for that purpose. It also helps
filter your deck by finding the next creature
you really want, making it effective in that
role.
The issue is that Foul Emissary is a 1/1 for 3
that requires additional set-up to be good. It's
only going to be as usable as emerge is as a
mechanic, and the verdict is still out on
whether or not emerge is going to have a role in
Standard. Most decks shouldn't need it, but it
might have a home after all.