How are Death Billiards and Death Parade related? I'm watching Death Parade right now and I just heard about Death Billiards, which seems to be a predecessor work for Death Parade. A one-shot of sorts, or so it seems. I'd like to know without spoiling the rest of D.P. how the two works are related, and if it's recommended to watch D.B. before D.P.

Update: After seeing Death Billiards, I'd say it's best to watch it after episode 2 of Death Parade, but before episode 7. D.B. is pretty much like the first episode of D.P., but with different bar customers.
Death Billiards was an OVA, and indeed a 'once-off' type of show.
It's very similar to the current series and you could probably drop it in as a longer episode.

Some of the surprises will have already been revealed in Death Parade, but it should still be entertaining as it's mostly focusing on the billiards, rather than what goes on behind the bar.

It's the same location and such, just billiards instead of darts or another game. It's not necessary viewing for Death Parade either as there aren't any plot points that haven't been covered by the first few episodes.