So there's a scene you can have with Gale at Moonrise Towers with trying to fix a broken moon lantern with dark magic. Gale says this as Mystra's chosen was his 'bread and butter' and is expert at this stuff. You can then point out he's the former chosen to which Gale says is charming and he hopes is due to shadowlands, it's really funny if you're romancing him because you can kiss after.

so the prompt...what if Gale is unknowingly right - that the Shadowlands IS affecting Tav mentally/personality wise? WHen fem!Tav taunts Gale about being Mystra's former chosen, it catches everyone off guard. Tav loves Gale, not to mention Tav is sweetness and light personified, it's actually rather sickening to some companions how *nice* she is. Typical heroic type. But when Tav starts having more sudden snide, mean outbursts the longer they're in Shadow lands, it becomes increasingly clear the shadowlands is having some effect on Tav, and Gale and their companions are determined to help her.

+ Tav starts having nightmares, anxiety etc that Gale comforts her from.
+ Halsin realises there's something (magic? shadow weave? possession?) that's affecting Tav and making her act out of character
+ Tav gets worse the longer she's in the Shadowlands
+ The only place Tav is free from this is Last Light Inn
+ Gradually it affects Tav more and more, it's not just being snide, it affects her health as well
+ Halsin has a crush on Tav and Gale knows - possibly Tav was previously oblivious as she only wants Gale
+ it's not a quick fix to help Tav
+ human or elf Tav prefered, Tav is also firmly good aligned. No Durge please.
+ Tav can't solve this herself - it's up to Gale and her friends to help her
+ During one of her bad moments Tav really flirts with Halsin non-stop - and even starts doing this in front of Gale
+ Tav starts forgetting after she has a nasty 'episode' this begins to really scare her - again Gale is there for her
+ Halsin and Gale have to work together and use powerful, dangerous magic to help her
+ Gale and Tav are a devoted couple - so he doesn't understand the flirting
with Halsin etc - essentially it's poisoning her mind and body.
+ Tav is normally close and kind with all companions etc but she starts being flippant/ snide to them
+ the shadowlands doesn't affect others like it affects Tav - perhaps she is targeted specifically
+ Astarion figures out something is wrong with Tav first