Rampant Dortmund netted five times in 15 first-half minutes with Shinji Kagawa grabbing two goals while Reus, back for the first time after six months out injured, completed the scoring in stoppage time.
Dortmund are top of Group F on 13 points, two points ahead of Real Madrid, who beat Sporting 2-1 away, with the two sides meeting in Spain in December to decide who takes top spot in the section.
"I was just happy to be there from the start," Reus, who missed the 2014 World Cup and Euro 2016 because of injury, told reporters. "It is how you imagine your perfect comeback.
"I waited a long time for this and so it is even sweeter. The ball will be signed by everyone and will go on the shelf."
With the 27-year-old Reus named captain and coach Thomas Tuchel resting key players following their league win over Bayern Munich on Saturday, Dortmund took control from the start.
But it was the visitors who struck first with Aleksandar Prijovic brilliantly curling in his shot with the outside of his foot in the 11th minute.
Their lead, however, lasted only six minutes with Kagawa nodding in Oussmane Dembele's low cross at the far post as Dortmund began their goal bonanza.
The Japanese grabbed his second goal a minute later and Nuri Sahin, making only his second appearance this season, got on the scoresheet after goalkeeper Radoslaw Cierzniak punched a Reus free kick straight onto the midfielder's chest in the 20th minute.
Prijovic got his second of the evening in the 25th minute and could have levelled when he chipped the ball onto the crossbar a minute later.
Two goals in three minutes from Dembele and Reus wrapped up a frenzied first half.
Reus picked up where he left off after the break, tapping in for his second goal in the 52nd minute with Legia's Michal Kucharczyk pulling one back yet again for the visitors.
The teams traded blows once more with Legia's Nemanja Nikolic scoring two minutes after Felix Passlack had headed in Dortmund's seventh goal.
Reus ended the goalfest with his third in stoppage time for the record-breaking 12th goal of the game.
(Reporting by Karolos Grohmann, editing by Simon Cambers)
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