To the right of the lower bonfire is the PvP-focused area (entering here will place you into the summoning queue for those wearing the Rat Covenant’s ring).
Inside you’ll find waist-high water, poisonous Dog Rats galore, Gryms, Grym Warriors and a Primal Knight . The whole lower and upper floors are covered with Pharros Lockstone faces and 90% of them are designed to activate traps for those in the Rat Covenant.
If you’re summoned to do battle, be wary of having to fight off the enemies described above (as well as attempting to kill the host player).
There are also a couple of small mounds that poke out of the water (offering better movement speeds); so make the most of these if you can.
On the lower floor you’ll find a Pharros Face in the water in the middle-right side of the room. Use a Lockstone on this and a door ahead will partially open up.
The quickest way to break it (as of May 2014); is to abuse a bug and hit a dead corpse with it over and over as hitting a corpse reduces durability 10x faster than any other method (tested on all platforms - including PC).
Up ahead you’ll find [2 x Amber Herbs] in a corner and through the archway is not only a ladder (it’s engraved into the wall face on the right); but a hidden room right at the end with a chest containing a [Faintsone] and a rare [Twinkling Titanite] .
There’s numerous Grym Warriors up here (usually standing on very narrow ledges); so lure them to more open spaces if you plan on fighting them.
Watch out for a wooden chest around the corner that’s guarded by a Grym Warrior . It’s a trapped chest but contains a [Titanite Chunk] and a [Petrified Dragon Bone] (so it’s worth the risk). And before heading up the steps look to the left for a cunningly hidden corpse holding a [Pharros Lockstone] and a [Soul of a Nameless Soldier] .
If you head left here there’s a room with a Bonfire just before a Boss fog gate. The opposite side of this upper floor only contains some souls and a hidden room with a trapped chest containing [15 x Magic Arrows] .
Rat Authority (Optional Boss Fight)
This boss can be extremely annoying as there’s four regular rats in here who’ll go super aggro’d once you get close (in turn triggering the giant rat boss to jump in too). The real trouble comes when they all gang up on you at once; making it very likely you’ll go toxic and die very quickly into the fight.
If you’ve got an area-of-effect spell/pyromancy then this’ll be perfect to swatting them all in one fell swoop. Otherwise, try and use a weapon with a sweeping attack as this’ll increase your odds of taking them out quicker as they try and gang up on you.
Once they’re out of the way you need to immediately turn your attention on to the main boss itself. If you’ve ever beaten Sif from Dark Souls 1 then you’ll have a good idea how to beat this boss.
- Your best bet is to stay directly underneath the rat for the duration of the fight (or for as much of it as possible).
- You'll find it easier to hit its legs if you **don't** lock on to the rat (as it'll only lock you on to the head).
- Magic or fire-related buffs or magic work especially well here and anything poison-related will actually do significantly worse (as the boss is a poison-based creature).
- Stay away from it if it throws up acid onto the ground as it'll break all of your armour, weapons and rings super quickly.
- You'll likely need to heal often if you're not spamming magic; so make sure to use Lifegems over Estus as you really want to keep moving where ever possible.