Though unassuming from the outside, Candulhallow's Tombstones is the site of a Bhaalist Crypt you need to enter as a key part of Orin's questline in Baldur's Gate 3. Like me, you might have explored the Undercity and made it all the way to the Temple of Bhaal before realising that there's no way to get inside. That's because you still need to complete the Investigate the Murders questline and meet the Murder Tribunal.
Once that's all done, you can get inside the Temple of Bhaal to try and save the companion Orin kidnapped, and fight for her netherstone. If you're still pottering around the Lower City, then the Stop the Presses and Thrumbo quests are both pretty fun, if you haven't tried them already. Otherwise, here's how to enter Candulhallow's Tombstones, find the door to the Bhaalist Crypt, and get the passphrase required to enter.
How to enter Candulhallow's Tombstones
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You can find this shop just to the north up the alley from the Basilisk Gate waypoint in the Lower City—you can recognise it by the tombstone sign out front. If you found your way here by pursuing the Investigate the Murders quest, then you should have picked up a key to the shop, but otherwise, lockpick the front door, and head through to the office.
There's a note on the desk that gives you a clue as to how to find the door to the Bhaalist Crypt. What you need to do is take the Butterfly on Skull painting off the wall, and push the button behind it. This will cause the cupboard on the opposite wall to slide across, revealing the door to the tomb. Now, when you interact with the door, you'll be presented with either a History or Investigation check, but all these really do is tell you to pursue the Investigate the Murders questline, since doing so will get you the passphrase you need.
The quickest way to complete the quest is to head to Elfsong Tavern and find the prospective murder victim, Cora Highberry, out front. You'll find her with a dwarf wearing red armour who's trying to poison her. If you confront him, you'll have to fight some shapeshifters and he'll escape to his next victim, Figaro Pennygood in the Facemaker's Boutique, right next to the Baldur's Gate waypoint. You'll be able to fight Dolor here and get the passphrase you need to enter the Bhaalist Crypt to meet the Murder Tribunal.