TV Line reports that Robert Knepper will reprise his Arrow role of William Tockman, aka The Clock King, in episode 7 of The Flash.
The Clock King is the first villain to crossover, and the episode will also feature a new villain, with two set to take on Barry Allen.
"We wanted to give Barry a real challenge… TWO bad guys!" Executive producer Andrew Kreisberg said. "[Barry Allen] will be fighting both a new enemy and one of our scariest and most successful Arrow villains in The Clock King."
Interestingly enough, the first Arrow crossover with The Flash is set to begin in each show's episode 8.
The Flash premiers Tuesday, October 7th at 8pm ET, with Arrow Season 3 debuting Wednesday, October 8th at 8pm ET.