It's actually a somewhat common randomly-dropped scroll.
You can always find it in the Spellhold in a room near the Lich (same area as the Vampire and the Kobolds).
You can buy it in the Underdark: Carlig (a duergar merchant) sells it. Also, you can buy it in Throne of Bhaal in Saradush: Lazarus Librarus sells it after you return his spellbook to him.
I think you have to be a drow to get it from Carlig. Carlig's inventory changes if you're in drow form, and visiting him after Adalon gives you access to some extra items.
I think you have to be a drow to get it from Carlig. Carlig's inventory changes if you're in drow form, and visiting him after Adalon gives you access to some extra items.
Never noticed his store changed... Probably because it didn't add anything significant.
Big, bad Skeleton warriors tend to drop one as a random loot, sometimes. They have also dropped ruby ray, sequencer, and horrid wilting scrolls in my games. Or Laeral's tears necklaces which are really worth the trouble of storing inside the gem bag and selling for quick gold. (Sells for 1800 gp or so)
Spell Trigger is also random loot, unfortunately, at least until TOB and Lazarus Librarus. I just came across a Spell Trigger scroll, though... in an all-Archer run, where nobody can use it.
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You can always find it in the Spellhold in a room near the Lich (same area as the Vampire and the Kobolds).
You can buy it in the Underdark: Carlig (a duergar merchant) sells it. Also, you can buy it in Throne of Bhaal in Saradush: Lazarus Librarus sells it after you return his spellbook to him.
But there is one that always appears in a chest in the Thayvian Enclave in Waukeen's Promenade. This is a part of Neera's SoA quest.