Undead bone shards: use 'em. Sooner rather than later.
Dark Souls 3: Undead Bone Shard locations and use guide
Dark Souls 3 allows players to level up bonfires to increase the efficiency of Estus flasks. Unlike past Souls games, you don't need to level up individual bonfires; in fact, you can only upgrade the Firelink Shrine bonfire. The good news is the effects are felt everywhere throughout the game world.
Levelling up your bonfire is a simple matter of obtaining Undead Bone Shards and then selecting the "burn Undead Bone Shard" option at the Firelink Shrine bonfire. There is no reason not to do this - you can trade one Undead Bone Shard to Snuggly the Crow for the Procine Shield, but apart from that you may as well chuck 'em all in the flames. If you burn every single Undead Bone Shard, you'll earn an achievement for reaching the maximum bonfire level.
If you have a low vigor build you'll soon find that a few upgrades will ensure one Estus swig fills your whole health bar - but don't run a low vigor build. Pile on that HP, yo; the returns on investment don't really peter off till you get to around 40.
Anyway, here's where you'll find every Undead Bone Shard we spotted on our run, in order of encounter. More precise directions to all of these can be found in the linked walkthroughs:
- Dilapidated Bridge: on a rocky platform you need to jump to, in the area where the giant's arrows land.
- Farron Keep: surrounded by slugs inside a tower near the door leading to Abyss Watchers and Farron Perimeter.
- Cleansing Chapel: on the balconies near the cemetery outside the cathedral, where the giant shoots arrows.
- Catacombs of Carthus: dropped by the first skeletal boulder; kill the nearby skeletal mage to destroy it.
- Smouldering Lake: dropped by the giant worm guarding the Old Demon King boss arena.
- Demon Ruins: on the bottom floor, which is patrolled by a couple of demons. It's near the flame slime in an alcove.
- Church of Yorshka: behind a tomb in the cliffside graveyard on the path towards Distant Manor.
- Profaned Capital: on a corpse right by the bonfire; also awards the Stretch Out emote.
- Dragon Barracks: on a ledge under the bridge leading to the two dragons.
- Grand Archives: on a snoring corpse near the final shortcut ladder by the broken window on the top floor.