So are there any good fics out there where a powerful enough enemy emerges
and this in turn forces Dumbledore with his Order to temporarily ally with Voldemort and his Death Eaters?

It could even be a returning Grindelwald, it does not matter, just as long as it is written well.

It would be funny to see The Dark Lord and Albus Dumbledore sitting in one room, coordinating strategies :awesome

 

jbern's Turn Me Loose is sort of close, with Harry (in a strained alliance with Dumbledore) and Voldemort joining forces at one point.

 

The Black Phoenix.

http://www.fanfiction.net/s/6125328/1/Black-Phoenix

Warning: Slash alert. I read this, not knowing that slash meant gay butt-sex. Still, no mention except mild references.

Gellert escapes Nurmengard.

 

Thanks guys, I'll look over these.

 

Becoming also features Order & DE allied. Be aware, it's slash but the idea behind the story make up for it.

 

It doesn't fit what you asked for at all really, and it is terribly short.

But Ravenous, by Enembee, has a short scene with Riddle and Hagrid teaming up.