You might as well go together at this point, you knew he wasn't going to be the ideal partner to begin with. Who knows, maybe you can use him as bait or something.

"Alright, come on let's go." You say.
"Hold on a minute." Warts says and finishes off one of the mugs that wasn't quite empty yet.
"Seriously?"
"Hey, I'm thirsty. Alright NOW let's go."

Warts and you leave the inn and after looking around a bit and scratching his head he beckons you to follow him. He walking just fine, so maybe he isn't that bad off.

You continue to follow without another word, until he gets to a secluded spot with a sewer covering.

"What the hell are you doing? You ask.
"Tryin' to take dis thing off, wat's it look like ya idjit? Help me over here." Warts says trying to take off the sewer lid.
"No, I mean why the fuck are we entering the sewers?"
"Cuz, it's the best way to get into Reginald's house. He's got a secret passage to it down here."
"How the hell you know this?"
"Cuz people talk shit all the time and not even realize ol' Warts is even there to lissen. Waz in a garbage bin and heard these two weird guys in red robes talkin about it in an alley awhile back."
"…okay I'm not even going to ask why the hell you were in a garbage bin, but weird guys in red robes, secret passages…are you sure Reginald isn't part of some fucking cult? I mean this is starting to sound a lot more complicated and dangerous."
"Well, why ya think I wanted someone else to come along?"
"YOU said this was for MY benefit!"
"Oh yeah…well it is! Look humie, ya gotta take big risks in dis profession. I don't doubt yer skilled. You've lasted longer than most independently in dis city. But I'm tellin ya, there's more out there that yer missin. I mean, sure it's foolish ta go head to head against da ECS, but robbin some rich asshole that may or may not be a wizard? Well dat's different."
"I really don't see how since both can get you killed. However, I do see your point to a small degree. I have been playing it a bit on the safe side. Still, if this is looking to be more trouble than it's worth then I'm getting the hell out as quick as possible."
"Sounds like a plan ta me."

You help Warts with the sewer covering and the pair of you descend into the smelly depths.

It actually isn't too bad, but that might be because you're standing next to Warts and his own stench is cancelling out that of the sewers.

Warts gets slightly more talkative as you walk to your destination. He mentions that the Thieves Guild has their hideout somewhere down here, but he doesn't know where. Neither does the ECS, since they'd probably mount an attack if they could.

Warts also goes on about how he's not fond of the ECS since a lot of them tend to have ties to Fel while he's dedicated to Yag. Says that worshipping Fel if you're a thief is like cheating since he's the god of shadows and all. Though he's not so much opposed to that, so much as he doesn't like all the dedication Fel seems to want. Yag on the other hand doesn't really require anything and he not might bless you anyway, but then he might give you a lot even if you don't do much. Warts claims that ALL the gods are like that, so he might as well pray to one that is at least honest about it all.

"Why worship any god at all?" you ask.
"Wat? You crazy or sumthin? You don't pick a god, youz gonna be in trouble when ya die! Even pickin' some stupid god like Karn would be better than nuthin! You don't believe in anything an' who knows WHERE yer soul goes! Nah, better to pick somethin' just in case."

It's a little surprising just how serious Warts takes religion, well as serious as a follower to the God of Luck and Chance can be anyway.

(It also occurs to you how it's weird how everyone's been getting religious on you lately)

"Eh, I guess I can sort of see that logic. I'm not really religious, but occasionally I have said the odd thanks to Yag." You say.
"That's all ya need man. He's lissening, even if he doesn't do anything." Warts says.

You and Warts continue a little longer and then he stops.

"Okay, think wez gettin' close. Should be a secret wall around here from what I heard." Warts says and begins pushing on the wall nearby.
"This seems a bit unprotected." You say looking around for potential traps.
"It's a secret door, maybe he didn't anticipate anyone finding it."
"You said the Thieves Guild is down here, surely they would have found it if this is their territory."
"It's a big ass sewer and they're the Thieves Guild not the Holgard Watch. Dey don't exactly patrol da place. They're all most likely up top breaking into shit. Besides, I told ya, they ain't the ECS. They aren't very territorial anyway. Dat's why we can walk around here without as much concern…well except for giant rats and such."

Just as you're about to ask Warts another question about the Thieves' Guild, Warts finds what you're looking for and when he does so a large bolt just whizzes over his head and into the wall behind him.

"Man, good thing Yag lissened to me." Warts says.
"Good thing you're short and constantly walk around slightly hunched over most of the time. Oddly, this makes me feel slightly better. If there weren't any traps at all I'd be even more paranoid about how easy this was. So far it seems like this guy just has shitty security, but then we haven't actually gone inside yet." You respond.

You and Warts both enter the passage and soon find yourself in a cellar with a bunch of wine barrels. Tons of them. Warts looks like he's hit a gold mine and immediately starts grabbing wine bottles.

"Hey, leave some room for actual valuable, you're supposed to be carrying shit for me, remember?"
"I am grabbing valuables! You realize how much old bottles of wine go fer?"
"Okay, point taken, just don't drink them all."

While Warts plays wine expert, you take a look around the room and find that there are stairs leading up and a solid metal door to your left, which is locked.

Since you don't want to go too far from Warts while he's alone with a bunch of wine, you stay in the cellar and pick the lock of the metal door, which is surprisingly easy.

You enter the dimly lit room and find what could only be a room dedicated to dark magical endeavors of some sort. There's two demonic looking idols starring at you and the pentagrams and symbols on the floor aren't any you recognize. There's also blood splatters on them.

From what little you now know of this Reginald, this guy is into some dark shit. Everything about this makes you want to just get the hell out right now, but you're already here now and you'd hate to just leave with wine bottles.

You walk carefully around the room making sure you don't step on any symbols or inside any circles just in case. You also keep an eye on the demonic idols, which still feel like they're watching you.

There isn't much else in the room itself, though there is another door off to the side, this one isn't locked and you open it.

However, you soon wish you hadn't.

You see a row of cages and various torture devices along with dead bodies in most of them.

"Fucking hell, I came here to rob some shit, not take a tour of a slaughterhouse." You say to yourself.

Just as you're about to shut the door a voice calls out.

"Stranger! You're not one of them! Not one those red robed freaks! (sob) Please! Help me! I can't die here, not like this! I watched my…(choke) just please…help. Who knows when they'll be back!" a nearly naked young girl in a cage begs. She reaches out to you desperately hoping that you'll rescue her.

You definitely didn't sign up for this…however given what you've seen here, you'd be quite the heartless dick if you just left her here to this horrible fate.

"Curse my parents for some of their morality getting through…" you say and head over to the cage.

The lock isn't that difficult to pick and you soon get it open causing the girl to not even wait around, she just runs past you, out the torture room and beyond. You suppose you can understand her not wanting to stick around, but a thank you would have been nice.

So much for good deeds.

Seeing as there isn't anything else in here, you return to the wine room where you unsurprisingly see Warts drinking.

"Hey, some humie gal just ran through here an into da sewers wearin' practically nuthin'." Warts says.
"Yeah and I'm glad to see that didn't interrupt your important work. Thought you were supposed to be helping." You say.
"Wat's yer problem? I am helpin'! Sides dese valuable bottles, I found out one of the barrels had a hollowish sound. Smashed it open and there was a body inside, idiots didn't bother looting it and he had some good stuff still on him." Warts answers and takes another swig from a bottle.
"Warts, you're telling me you found a body in one of these barrels and you're STILL drinking the fucking wine?"

Warts suddenly stops drinking. He ponders your words a bit and then sniffs the top of the bottle. Then he promptly gives a little shrug.

"Still tastes alright ta me." Warts says
"Of course it does. Look, we're getting the hell out of here. This place has been a big fat zero." You say.
"What about all these valuable bottles of wine? An' ya haven't even checked da upstairs yet!"
"Yeah well I've checked enough of the basement to know that most likely there isn't going to be anything that precious up there that's going to make me want to stay another moment in this fucking house. There's some dark magic going on here and I'm not sticking around for it rub off on me."
"Just say a prayer ta Yag, you'll be fine."
"And as you like to point out, he might not listen."
"Fine, we can leave if you really wants, but I'm just sayin' we haven't gone through the entire house yet."

Funny how Warts is content on letting you do all the exploration. He could have easily been searching the upstairs while you were stumbling upon sacrificial captives.

You search the rest of the house

Well in for a copper in for a trove.

"Fine, but whatever I find I'm keeping. You stay here and continue getting drunk seeing that that's what you're good at." You say and make your way up the stairs while Warts continues to do what you suggested.

You get to the ground floor and begin to search the downstairs cautiously and methodically, all the while grabbing whatever looks valuable. By the time you're finished, you've more or less filled your pack, you're starting to think that Warts had the right idea after all, but you decide you've pushed your luck enough and that it's time to go, even if there is a second floor to loot.

As you're making your way back down the stairs, Warts suddenly is running up them.

"Warts, what the hell?" you ask
"Up! Go back! Up! We gotta… HRUP!"

Warts vomits all over your legs and boots. You're about to yell at him, but then you soon see what's caused all this excitement from him in the first place. Several people in red robes are shouting and running after him, and now most likely you too now that they've seen you.

You immediately turn around and run back up the stairs, though Wart is busily trying to get past you. In between that, the heavier pack and the vomit still covering your boots, you end up slipping and tumbling down the stairs and into the cultists.

While the cultists cushion your fall down the stairs, it doesn't do much on improving your situation overall. There are enough of them that after the fall to the bottom, you're immediately nabbed, beaten and stripped. And not just your of your weapons, you're stripped of your all your clothing and throw you into one of the cells that you rescued that girl. They're even less pleased when they learn that you released her.

You aren't killed right away, but after a couple of weeks you wish you had. By the time you're sacrificed, you're a babbling mess that is grateful for death.