Get away from that path, we’re going back the other way.
Dark Souls 3: Crystal Sage to Farron Keep
Oh, what’s that? You thought we were going onwards from the Crystal Sage? Wrong! There’re more swamp adventures to be had. Turn right around and go back towards Crucifixion Woods.
There are two paths forward after Crystal Sage, and there are good arguments for proceeding to Cathedral of the Deep before Farron Keep. It’s the more obvious path, for one thing, and many of the loot drops in Cathedral of the Deep seem intended to aid you in exploring Farron’s poisonous swamps. Nevertheless, we suggest heading through Farron Keep first, because Cathedral of the Deep is a bag of dicks.
From the Crystal Sage bonfire, Return to the building with the two spellcasters and clear out the nasty central room; it’s made easier in this direction by the location of the powerful spell caster, who’s just begging for a backstab.
Go through the central room and take a left before the bridge. Go down the path where you fought the Crystal Lizard, then head through the door where the grunt ambushes you. There’s a set of stairs here; if you look up, you’ll spot a powerful spell caster and a grunt. When it’s clear, you can climb the stairs to the top, go out on the balcony, and drop down to a lower ledge to collect a Ring of Sacrifice. Drop down to ground level again.
Walk out onto the platform and then down into the swamp so that a king crab charges you. Let it get close then dash back up onto the platform; it can’t get you here. Do enough damage to it, and it’ll burrow down into the ground, giving you a few minutes freedom from sudden death by crap. You can then proceed into the water to grab the nearby Green Blossom, killing a bunch of crabs on the way. Nearby, there’s a small door from the swamp leading under the building you came from. There’s a crucifixion victim in here. If you can beat him, or lure him out then sneak in, there’s a full set of Sorcerer’s gear and a Sage Ring.
From the platform where you fought the crab, explore the swamp in a clockwise direction to grab a full Fallen Knight gear set. On the land here is Large Soul of an Unknown Traveller – and a pair of very nasty player-like NPCs. If you get close enough to aggro one, both will come for you, which is highly undesirable.
Use an arrow to bring the first NPC over. Take him down, and then the second NPC. This is tough, but necessary. You’ll get the Exile Greatsword and a club weapon for your pains.
Update: commenter DTB suggests cheesing the NPCs by running past them and climbing onto the ladder. Apparently they’ll often fall over the ledge in their attempts to reach you, and fall to their deaths. Just be careful not to fall down yourself!
When the pair are down, you can wander back to the crab platform, climb the stairs where you fought the powerful spell caster and shield grunt, and take a path at the middle level to drop down a series of ledges so you can collect the Golden Falcon Shield. (You can do this any time, but you don’t want to risk aggroing both enemies at once by getting too close.)
Before you pass through the doorway the NPCs were guarding, you may want to venture into the swamp to collect the Great Swamp Pyromancy Tome. It’s at the base of a tree near the wall to the right of the doorway as you face it, guarded by a giant crab. Drive the crab to burrow a few times, and make a dash for it. Return this item to Cornyx to unlock new pyromancy spells.
When you’re ready, head through the doorway the NPCs were guarding and collect the Homeward Bone on the balcony, then descend the ladder to activate the Farron Keep bonfire.