The film centers on Stan, a shadow (Bill Hader) who yearns for a more exciting life than being attached to a timid human named Stanley Grubb (Josh Gad). When a crime in the shadow community puts both of their lives in danger, Stan is forced to take control of Stanley, thrusting both of them into an adventure featuring a shadowy villain, who intends to lead a rebellion to take over the human world.Stanley saves the day when his shadow is taken away by the shadow villain.
Tom is listed as the voice of the “shadow villain” in this wiki. However, keep in mind that this wikia page is fan-run, so this is pure speculation for now. The only official announcement(s) about this film have not listed Tom’s name:
So, once again, take this as pure rumor mill until an official announcement is made. Having said that, this rumor is not so ridiculously far fetched as he has done voice over work quite often in his resume/CV, and one of DreamWork’s founders is Steven Spielberg.