You're not supposed to know what's in the light
The light, locked away at the top of the lighthouse, is an object of fascination from the start of the film. Thomas refuses to let Ephraim care for the lighthouse's lens, relegating him to grueling manual labor, and we briefly see why. When Thomas is alone in the blinding beam, he experiences a sort of euphoria. Is in the throws of religious ecstasy? Is it sexual? And what's the deal with those octopus-like tentacles oozing goo? "It's a moment of mystical love, love for this thing that cannot be explained," Dafoe says. He continues: "He's toasting to his love because he's a believer and that is what he's dedicated his life to. That's the simple explanation. When you're looking at that light, you're communing with something that's not human, that's beyond human, that's eternal."
Ephraim's mania that sets in over the course of the film is partly borne out of deep guilt, given how he ultimately reveals that his name is not Ephraim, but Tommy, and that he was responsible for a man's death when he was a woodsman. But the promise of what the light contains also contributes to his growing insanity. He ultimately kills Thomas, and storms up to the lighthouse's peak to see what's within. Inside he's met with a blinding flash, and his face contorts. He falls down the steep stairs. In the movie's final shot, we see seagulls eating his innards.
Eggers was never tempted to reveal what Ephraim/Tommy sees. "The movie was always intended to have a level of obscurity and ambiguity because that's what I enjoy," Eggers says. "Sometimes a simple story well-told is the best thing, but that's definitely not what we were after."
Pattinson shot his moment in the light at the very end of production, but doesn't have a clear answer for what Ephraim/Tommy was seeing. "I always thought of it in such a sort of reductive way," he says. "There's something about wanting something to be real so much and you are having a transcendent experience. And regardless of whether the thing has a mystical power or not you could still wind yourself up into that frenzy." He partly was riffing on a note in the script, and partly just went for it. "I think in the script there was a mention that it was so pleasurable it becomes absolutely terrifying," he says. "I sort of really liked the idea of that. I really connected to it. We shot it right at the end, after having done such an enormous build-up to touching the light. I probably had gone just a little bit mad by the time I did it."