When in doubt, have a man come through a door with a gun in his hand.

When in doubt have a man come through a door with a gun in his hand. This could get to be pretty silly, but somehow it didn’t seem to matter.
Raymond Chandler, The Simple Art Of Murder

It’s always good to have a spare enemy you can introduce Chandler style in case of emergency.

I did that tonight for the first post-holiday session of Blood And Smoke, introducing two new players’ characters, bringing forward a so-far minor subplot, and bringing in two guys in a van armed with MP5s and crucifixes. Why were they there? Good question. One the PCs aren't cleared for.

What constitutes a good drop-in group?

An entirely new enemy, who wants the MacGuffin, or wants to kill the PCs, or indeed wants to kill the PCs’ enemies.
An existing faction or a new group connected to an existing faction. This means you just need a reason for them to crash in rather than having to create them.
An enemy of an enemy, an established contact, something that one or more of the PCs have in their backgrounds which has never appeared before.
Ninjas.

As noted on the TV Tropes page for Chandler’s Law: The Dungeons & Dragons adventure Whispers of the Vampire’s Blade suggests using this trope, and includes a table of possibilities for who exactly is kicking down the door and why.

(In related Vampire news, Justin Achilli proofs Anarchs Unbound live online.)