User blog comment:OH&S/Toaru Majutsu no Index VS Durarara!! Special Collaboration/@comment-24.189.131.124-20150124060658/@comment-4419516-20150124160702

From the few spoilers from animesuki:

The second part (by Narita) Izaya tricks Accel into fighting Shizuo with help from Loki (who disguises him as a member of the Board of Directors). Izaya convinces him he's a threat to the Sisters and LO. Shizuo only gets a headache from Accel's bloodflow reversal. When he tries throwing a vending machine Accel refelcts it only for him to punc it back so they do this back and forth until it breaks. Because Shizuo can't actually do shit against Accel he starts getting beat around. Celty sees this and gets mad, trapping Accel in a shadow sphere to suffocate him and calm him down. Accel uses his wings to rip himself free. He admits that Celty could hacve killed him if she wanted to because shadows don't have density and are more a phenomenon than a physical material.

The crossover is apparently caused by the Absurd (A Simple Series)