Speak with Leah in the Hidden Cellar under Adria's Hut. She gives you the key, and her mother's journal. What she has seen of it is disturbing. She wants to know more about her, but that can wait. We need to save Uncle Deckard first. You tell her to stay here and study her mother's secrets, you may have need for them. You will take care of finding Deckard Cain.

Adria's Journal is added to your own journal:

Aidan came to me last night. I suspected what was lurking with his troubled heart. I consoled him as best as I could. Regardless, wherever he's headed, Hell will surely follow in his wake. The shadows close in on Tristram once again. But, like Aidan, I'll be gone before they fall. -Adria.

Your objective now is to go to the Cathedral:

Go to the Cathedral by exiting the Hidden Cellar and following the path northeast from Adria's Hut.

"I know that Leah will be of little assistance to me as I descend into the cathedral to find her uncle. If she stays here, she may be able to learn more about her mother, a mystery that I can tell is of great importance to he."

Climb up the ladder to leave the cellar, then exit the hut. Be prepared for a possible Risen ambush. Go Northeast, search for a chest in the trees nearby. Keep going NE along the trail, up some stairs and crossing a bridge.  You might come across a mass grave on the way.

Diablo 3 Mass Grave

This grave is filled with undead and risen, but also with treasures and chests, so it is worth it for you to take a detour, both for the experience and the loot. After walking north east some more on the Old Tristram road, you finally arrive at the Tristram Cathedral. The fallen star lies within.