An optional boss, a friendly companion and a stack of sweet loot. Who could ask for more?
Dark Souls 3: Cliff Underside to optional Demon boss
This optional boss fight and treasure un is well worth doing. You'll find some great loot, and also advance an NPC quest. Just follow the directions here, or see the NPC guide for more information (and spoilers).
Die or fast travel, then return to the church where you spoke with the giant. Take the lift up, but run or roll off when it passes the central platform to access the middle level.
You'll find Siegward of Catarina on an external ledge here. Chat with him, then head down over the rooftops to fight the optional Demon boss (or sneak past it by hugging the left edge)
The fight’s not massively hard if you’re doing okay with extended battles. The only move you really need to watch for is when the boss leaps and brings its weapon straight down; this will really hurt. When the boss stands up extra tall and winds up, it’s going to do an extended fire breath move. You’ll be fine if you’re behind it or circling, and it is a great opportunity to go wild with attacks. Keep some stamina for that jump attack, though; it loves to follow up with that. Siegward will join the fight, and he’s a real help here; he chips away at the boss’s health pretty respectably, but also he holds it attention if you need to back off.
If you struggle with this battle, it's easy to cheese; shoot arrows at the boss before you drop down off the roof to ground level, and you'll be able to attack it in safety while Siegward takes the brunt of the action. Don't let him die!
If you survive the battle, you’ll score a Fire Gem and the Soul of a Demon. Speak to Seigward afterwards to receive a Siegbrau and the Toast emote, too.
Whether you use force or stealth, you can collect a Homeward Bone from the cliff near the overturned carriage. With luck you can also sneak to the Large Club on the far side of the area the Demon is patrolling, but it’s not worth the risk if you’re nervous about it. If you killed the boss making the area safe, climb up on the little peaked roof tucked under the eaves of the building on the opposite side of the fire from the church you came from. You can cut down two bodies for a Pale Tongue and a set of Northern gear. Take the Pale Tongue to Leonhard in Firelink Shrine (he’s next to Lothric’s throne) later to receive a key.
The path out of here lies through the only building you can enter. You obviously don’t want to walk under all those cages, but somehow none of them are alive, so grab the Red Bug Pellets at your leisure. Continue up and around via the stairs. There’s a short hallway on your left with an item glow at the end; there are two living cages on the ceiling, so don’t fall for their wiles as you grab the Alluring Skulls.
Continue on over the bridge. There’s a living cage immediately to your left in the next room. Two dogs will attack from the right as you advance into the room. Be very careful entering their hidey-hole and taking the stairs up; there are four living cages above this side room, and two Evangelists waiting to attack on the level above.
Update: commenter DXMKiller pointed out we missed a chest in this area; open it to grab some Human Pine Resin.
Head on out to the balcony and up onto the roof, taking the ramp on your left to grab Flynn’s Ring. Walk around to the far side of the structure and look for a platform you can drop down to, and from there onto the circular tower. There’s a Homeward Bone on the opposite side and an arch leading to the tower’s interior.
If you have Spook, now’s a good time to apply it, because you’ll be dropping down from platform to platform and a miss is fatal. You’ll need to run forward from the planks on the left to make the narrow ledge on the other side, and thence to the ground. You can collect a Mirrah gear set at the bottom. Roll through the suspiciously glowing crates to collect a Chloranthy Ring, too.
Exit through either arch and drop down in front of the large locked doors you passed earlier. Make your way to the bonfire of your choice to continue; we're headed for the boss arena via Dilapidated bridge.