Previously the most goals scored in a Champions League match was 11, with Reus' third goal setting a new Champions League record of 12 in one match.
Germany forward Reus, who missed out on Dortmund's Der Klassiker triumph over Bayern Munich last time out with a heel injury, had not featured for Thomas Tuchel's side since May prior ace 16 purecontrol to Tuesday's clash at Signal Iduna Park.
And he wasted no time in returning to goal-scoring form X 16+ Purechaos black as he sealed his hat-trick in second-half stoppage time.
It was Legia who took the lead through Aleksandar Prijovic early on, but Shinji Kagawa's double and Nuri Sahin's header put Dortmund well in control.
Ousmane Dembele added Dortmund's fourth after Prijovic had snatched X 16+ Purechaos gold another for Jacek Magiera's side, with a Reus ACE 16.1 Primeknit double coming before Michal Kucharczyk pulled another one back for Legia.
Felix Passlack's header made it seven for Dortmund, before Nemanja Nikolic netted Legia's fourth - Reus on hand to set the new Champions League record with his final goal.
Despite their lowly position in the group, Legia - who drew 3-3 with Real Madrid on matchday four - started brightly and were rewarded when Vadis Odjidja-Ofoe took advantage of some nonchalant Dortmund defending to tee up Prijovic, who proceeded to curl in a wonderful finish off the outside of his boot.
However, the hosts swiftly restored parity, Kagawa superfly mercurial v cr7 black ghosting in behind Legia's static back-line to head home Dembele's cross 17 minutes in.