Help finding an appropriate monster for my dungeon?

[Whiplord]

WARNING: IF YOU ARE ONE OF THE PLAYERS IN MY ASH FALLS GAME, PLEASE DON'T READ AHEAD. KTHX.

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So, I'm running a campaign that's currently in Sharn, and I'm making a small dungeon crawl in the sewers. The dungeon is actually the base of a small cult, and they've mostly cleared out the space of any native monsters. However I'm picturing one monster who keeps on eluding them, or they just haven't bothered disposing of it yet. Whatever it is I'm picturing stealing random items from around the dungeon, including a key that the players will need. I'd like it to be around CR 6.

I've been combing through my books for awhile now, and am having trouble coming up with an appropriate one.

Thanks for any help.

[reptilecobra13]

A yuan-ti abomination (MM pg. 264). It's chameleon power ability would definitely help it avoid detection, and it's a stable CR 7 creature on its own. Additionally, it could be connected to the cult if you so desire.

[WildPyre]

Hmm maybe something with levels in rogue or one of the other sneaky classes? A kobold or goblin perhaps. I'm not versed on the setting so I'm not sure of what exactly Sharm is like.

[Runestar]

Pseudodragon with class lvs? At cr6, you could have a pseudodragon sorc6 (using non-associated class rules).

[Saintheart]

Ethereal Filcher from the Monster Manual? It's only CR 3, but if you gave it some class levels ... it's designed as a thieving-by-nature creature.

[AslanCross]

Hmm maybe something with levels in rogue or one of the other sneaky classes? A kobold or goblin perhaps. I'm not versed on the setting so I'm not sure of what exactly Sharm is like.

Sharn's the largest city in Eberron. It consists of lots and lots of extremely tall towers (as in as tall as modern skyscrapers) and has a long, complicated history (as such it's got really deep ruins and tunnels beneath its sewers). The industrial sector beneath the city is usually manned by kobolds and goblins, so I'd go for a kobold or goblin rogue, actually.

[WildPyre]

That would explain it... never actually played in Eberron. Did a lot of FR, some Greyhawk, and a lot of custom worlds... never played in Eberron though.

[Roc Ness]

Pseudodragon with class lvs? At cr6, you could have a pseudodragon sorc6 (using non-associated class rules).

Instead of Sorcerer 6, how about Swordsage 3? Thanks to the hd/half initiator level thing, you get second level maneuvers, granting you Shadow Stride (teleport 50 ft) and Cloak of Deception (Turn Greater Invisible for a round). Perfect for a thief.

Swordsage also nets more skill points, and access to the Shadow Hand feat, so that the Pseudodragon will be useful in combat if it is cornered. Which it probably won't be. D'you want me to stat it out?

[Kol Korran]

some nice ideas allready, here are a few more:
1- how about an advanced mimic (1-2 levels of sorcerer or rogue or whatever). or you could go with phasm too. both could have seen lots of things in the sewers and about the cult, and might dispense information (perhaps after a fight) about them. they could be interesting recurring characters even.
2- a changling rogue could do well too, perhaps half rogue, half wizard?
3- a wraforged scout, who usually travels underwater, and goes where no one else can. rogue/ scout?
4- perhaps a couple of Otyughs, or one advanced one? the cult uses them as waste disposal, and to intimidate prisoners. sure, they aren't exactly thieving types, more the murdering types. (though the cult might appease them with victims)
5- a small band of theiving Kenku (forgot which MM. either 2 or 3). a flock of killer bird people, who are on some uneasy truce with the cult at the moment.
6- a slightly advanced Phase Spider, who keeps attacking/ thieving (you could change his feats to reflect that), before disappearing and reappearing in an adjacent corridor. might be a very frustrating fight, but could easely explain why the cult hasn't finished it off.

or better yet, find some way to mix two of the above together:
- A mimic working with an otyugh- one provides the raw power, one provides shenanigens and trinkets.
- a couple of changlings working with a kenku band.
- a warforged scout working with a phase spider, a strange partnership forged in the sewers of Sharn.
- any other configuration, perhaps with monster suggested by others like the ethereal filcher, the pseudo dragon or a master kobold/ goblin sewer bandit.

perhaps i'll think of something later