Undead Parish is an area in Dark Souls that carries over from the Undead Burg.
Meanwhile, we have explainers on how to access DLC Artorias of the Abyss, the best gift and best class in Dark Souls, Dark Souls' best weapons, Dark Souls Covenants, Dark Souls Estus Flasks locations, Dark Souls Titanite Slab locations and NPC quests including Lautrec of Carim.
Now that you've arrived in the undead Parish, take out the Hollow Soldier and walk up the spiral staircase to encounter another Black Knight.
This has a greatsword, so do be careful getting stunlocked in its chain of attacks, again try to backstab or parry this one as much as possible for a Titanite Chunk. With that out of the way, head down and up the stairs for a very challenging room.
Immediately sprint to the right and up the stairs as soon as you enter. Two sword hollows will fall you up the stairs, deal with these first, then there are two archers and another hollow out to get you, make sure you use your ranged skills to deal with the archers on the bridge.
There are some alluring skulls down here, where you should be able to aggro the armoured Boar into the pit of fire. Do this and backstab it in the rear in order to get rid of it fairly sharpish. Use Firebombs or Fireball to sort it out. Then, head down.
You will see a hollow running away, but don't be tempted to chase after it- there are a lot of hollows who aggro easily here, so keep it slow and steady until you've cleared them all out.
To the right of the room is a Mystery Key, something we're going to save for later. Head up the ladder and onto the beam towards an item. Here, you'll find a Large Soul of Lost Undead. Head up the other ladder and traverse the fog gate. From here, take the right path to fight your first Balder Knight.
These ones can parry you, so be sure to watch out when its sword moves into a different position so you know when to wait. Defeat this, and get a Knights Shield on a corpse. Then, head back and to the left path and up the stairs. Then, look immediately left and deal with the hollow soldier in your way. Continue on the path down, then to the right you will find a spear hollow, take this out quickly, as a Balder Knight will come to ambush you. Take these both out and head left from where you emerged.
Continue until the path splits. Here, you can go left and to the front of the Parish. Take out the Balder Knight and get a halberd. Head down the stairs and towards the gate for the Basement Key, you'll need this to access a future area. Now, lift the gate from the lever to the right.
Go back to where the path split off and head across the wooden bridge. There are a few hollows here, but nothing that you shouldn't be able to deal with. Then head right down the long corridor. Head down the flight of stairs to find a bonfire. Rest here if you want, and head out the doorway. Here, you will meet Seigmeyer of Catarina for the first time. Head back, and down the stairs again to meet Andre the Blacksmith. If you head down, you will encounter a Titanite Demon. Let's head back up and back to where you fought those hollows in the Parish.
Here, you'll find a giant armoured knight. Lure it into the courtyard and kill it for a titanite shard. Its attacks are very easy to telegraph, so you should have very little trouble with this one. Now, head into the elevator on the right. After a lengthy ride down, you will be back at Firelink Shrine - good to know! Now, head back up and underneath the statue, will be a Fire Keeper Soul. Hang onto this for now.
Turn around, and take out the Balder Knights you see in front of you. Lure them into the courtyard to get away from the magic attacks from above. Enter back into the Parish, and head up the stairs to find another Balder Knight. Firebombs work well here, due to the tightness of the area. Carefully walk up, then before entering, 10 hollows will attempt to rush you. Again, any AoE firebomb or spell will have these down in no time, don't try to melee them.
Once in the room, take out the channeler, and from where he was standing, go forward and left. Continue down the Hallway, killing the Balder Knight along the way. Loop all the way around to a dead end, destroy the barrel with the corpse inside to get a Humanity. Now, head back to the middle section and destroy the wooden door that has light shafting through.
Head inside and up the stairs to encounter Lautrec of Carim. There is a choice to be made here - do you free him or not? Without spoiling here, the prior link explains what happens if you do or don't!
Head straight up, and past the main room to get to a corpse with a Large Soul of Lost Undead. Then, go past where the Channeler was standing to go left and up the ladders. Here, there is fog gate that leads to another Boss Fight - the Belfry Gargoyles.
Where to use the Basement Key
Once you've killed them, head up the tower and ring the first bell. On your way down, you will encounter Oswald of Carim, an NPC and merchant who sells some pretty interesting items. Head back to where we first met Solaire of Astora, and you will recall a locked door.
You will be able to unlock this with the Basement Key we got earlier, and into Lower Undead Burg.