Spoilers for Act 3, by the way.
- Get to Act 3 and start the Murder Tribunal Quest.
- Separate your party to have the main thief be separated and invisible.
- you want this character’s sleight of hand to be buffed to the max, Smuggler’s Ring, Gloves of Power, Graceful Cloth/Enhance Ability (Dex), Guidance, Bardic Inspiration, Proficiency, Expertise, Shapeshifter’s Boon Ring, etc.
- Speak to the Bhaal Cultists and seek judgement from the Tribunal.
- Once the spirit is summoned (such as Dribbles the clown) you can move your invisible thief up to them and pickpocket them.
It is a DC 29 check, so going Rogue 10 for Reliable Talent and having +8 proficiency is only going to get you to 18. Smuggler’s Ring(+2), Guidance, Bardic Inspiration, Gloves of Absolute Power and Shapeshifter’s boon Ring give you a minimum of +1. Minimum total increase of +6, so we’re at 24 minimum now.
Graceful cloth gives another +1 Dex modifier, and you can max you Dex with ASI, giving you another +5.
In theory you don’t need all these items, as Reliable Talent gets you a minimum 30 with all these items.
Be warned, as after the thieving is done, combat will start.