Smallville Season 10 Episode 5 (10/22/10)
Having inadvertently taken an artifact from the dig in Africa, Lois became possessed by the spirit of the Egyptian goddess Isis, and attempted to resurrect her lover Osiris using Clark's body.
Kat witnessed "Lois" doing superhuman things and tried to out her as The Blur. Clark destroyed Kat's phone, and the photographs she had taken of Lois.
Tess helped Oliver and Clark figure out how to free Lois from Isis' grasp, and as a result they entrusted her with The Watchtower. Tess was overcome with emotion at the gesture and had to leave the room.
Tess decided not to have Alexander, who is growing at an accelerated rate, sent off with the psychologist, but rather keep him at the Luther mansion with her.
Clark finally told Lois the truth, that he is The Blur. She exuberantly tackled him into a stack of papers and asked him "What took you so long?"