“Babylon” is episode eight of season nine of Stargate SG-1.
SG-1 are searching for some legendary Jaffa warriors who have apparently been independent from the Goa’uld for 5,000 years. The team come under fire from opponents that cannot see, and make their way back to the gate. Mitchell stays behind, covering them, and is attacked. He manages to defeat the Jaffa who attacked him, but is still captured.
SG-1 recover the other Jaffa and manage to save his life. Because they think Mitchell killed one of their own, the Jaffa sentence him to death in combat, but first he needs training up.
There is a Prior of the Ori interfering with things, which is why the Jaffa attacked in the first place.
What might be intended to be a surprising reveal was in fact completely obvious.