From Wikipedia:
Scarecrow briefly appears in the fourth issue of the Blackest Night storyline. His immunity to fear (brought about by frequent exposure to his own fear gas) renders him practically invisible to the invading Black Lanterns.

However, his current status of fearlessness has taken a further toll on his sanity, exacerbated by the long disappearance of Batman: he craves for fear, exposing himself deliberately to the revenant army, but knowing that only Batman could scare him again.

He again appears in the sixth issue as he is deputized into the Sinestro Corps for twenty-four hours in order to combat the Black Lanterns.

From Wikipedia:
During the Blackest Night event, when word got out that apparently everyone around the world who have died are rising as undead Black Lanterns, Lex Luthor isolates himself in his safehouse in fear that all the people he had murdered over the years would also reanimate and seek revenge on him.His fear is justified as his victims, including his deceased father, arrive, seeking to feast on his avarice-filled heart.

However, Luthor escapes after receiving a power ring fueled by the orange light of avarice and becomes a deputy of the Orange Lanterns. Luthor arrives at Coast City and joins the battle against the Black Lantern Corps. Luthor engages battle with the Black Lantern versions of Superman and Superboy. However, the Agent Orange Larfleeze wants Luthor's ring off, as the alien do not want to share his power with him, resulting them battling each others for it.

From Wikipedia:
While Ganthet himself becomes a Green Lantern, Barry Allen is chosen as a deputy Blue Lantern, Mera as a Red Lantern, Lex Luthor (who had been battling the hundreds of people he had killed over the years) as an Orange Lantern, Scarecrow as a Sinestro Corps officer, Atom as an Indigo Tribe member, and Wonder Woman as a Star Sapphire after she is broken free from the black ring's influences.