Assuming two Spanish clubs cannot be in the same group, the group of death could have: 2015-2016 Semifinal loser Bayern Munich 2015-2016 second place Atletico Madrid 2015-2016 Round of 16 loser Dinamo Kiev, who finished ahead of Porto in the Group Stage and got a Round of 16 draw at Manchester City 2015-2016 Round of 16 loser PSV Eindhoven, who advanced from a group by finishing ahead of Manchester United and then did not allow a goal in the Round 16 until they were eliminated by Atletico Madrid on penalty kicks
LOL @ Kiev and Eindhoven. That would be a really dull group as Bayern and Atletico would advance without breaking a sweat. You could actually do much better than that for a true group of death. Tottenham Hotspur will be in Pot 3, and they're 66:1 to win the whole competition which leaves them tied for 10th compared to 300:1 for Kiev which is tied for 23rd. In Pot 4, you could end up with an extremely tough Roma team who's the second favorite in Serie A, and is only listed as high as 100:1 to win the Champions League since they'll probably be unseeded in the playoff round and could have to face Manchester City, Porto, or Villareal just to make the group stage.
Worst possible group of death heading into the draw: Bayern Munich Atletico Madrid Tottenham Hotspur AS Monaco The chances are we probably won't get a real group of death though, as Spurs are the only legitimately scary team in the bottom half of the draw, and of the non-English sides they can be drawn with in Pot 2, only 2 out of 6 are really big clubs. Then, even if Spurs get drawn with Atletico or Dortmund, there's still a chance, they get out of jail by drawing CSKA Moscow or Benfica in Pot 1. I mean Benfica's a very solid side, but they're not a real contender to make the semis, let alone win, and no one's really gonna care if they get knocked out in the group stage.