It's definitely a difficult task, not because the Arcane Trickster is bad, but because Thief is so amazing.
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My first thoughts:
Sleep is a powerful choice. Especially if more than one character has it, allowing you to spam the spell in combat. I think you'll find there are times when Astarion will rather do that than take a sneak attack.
Personally, I like Disguise Self for Astarion. It works in a roleplaying sense because he might want to hide from vampire hunters, and it's also nice because Astarion gets Speak with Dead for free if you give him the Necromancy of Thay book. Being able to disguise himself allows Astarion to question the dead, even if the party caused the death in question.
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I like Find Familiar for Astarion. The raven has excellent mobility and can help Astarion get sneak attacks. As a side note, it's always good to try to get the familiar an extra five hit points from an Aid spell, if possible. Makes it a bit more robust and long lasting.
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I wouldn't worry too much about his Int. as long you don't pick spells that lean heavily on the attribute. Which means there's probably no reason to waste his helmet slot on the warped headband of intellect.
That said, it might be worth getting the Circlet of Fire and taking something like Chromatic Orb. That way you can throw out a fire spell and get a bonus action, which translates into another off hand attack for Astarion. Either a second melee weapon or a second hand crossbow.
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Note: it's worth getting the Spellthief bow for him, in my opinion, even if you change it out for two crossbows later. I mean use it to get a spell slot back and then switch back over to the two hand crossbows.
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In my opinion, your feat is probably best spent taking the moderate armor proficiency and raising his dex by one.
This allows him to use a shield and wear something like the githyanki armor for a 19 AC. Or maybe the dark justiciar armor that causes damage to attackers when he's partially obscured.
For Astarion, I tend to prefer using a shield to having an off hand melee weapon. Mostly because I like his AC being higher, and I can always get that extra off hand attack with a second crossbow.
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I've heard good things about Fog Cloud, but I've never found myself using it all that much. It's definitely good for hiding, as long as you're actually hiding in it as opposed to just standing in it.
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So, I think I would outfit him something like this, maybe:
1. Helmet: Circlet of Fire or Helmet of Grit
2. Amulet: Broodmother's Revenge (Kagha's amulet) --or the Sapphire Spark if you chose Magic Missile
3. Armor: Githyanki Armor
4. Shield: best available, whichever
5. Ranged: two hand crossbows (and Spellthief from the Hag)
6. Melee: magic rapier, eventually Nere's
7. Boots: Speedy Lightfeet or maybe even the Linebreaker Boots
8. Gloves: Gloves of Power (if you're branded) or Fleetfingers for the extra jump after a dash
9. Rings: Smuggler's Ring and something else, maybe the Invisibility Ring later
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This seems like a solid build to me, but it's pretty basic and might be lacking when it comes to flair.
+1
