I'm pretty sure everyone who crossed his road in BG1 remembers his meeting, as it was particularly... hot I guess is the word. He is described as a level 9 mage, and he does not appear anywhere else in FR lore. However there are a few spells named after him, three of them to be accurate, that you can find in Shadow Keeper
There are many such spells, particular to this or that character you cannot control, but they usually are not given a description, while these spells have a description.
There are three of them. One is mostly forgettable, it add slight physical bonuses, not so useful, but the two others are just the most powerful spells in the game:
Daer'Ragh Aura Cleansing: a short-cast-time level 4 longer-lasting improved alacrity
Daer'Ragh Mental Agility: reduces cast time by 5 ( with vecna and amulet of power that s instant cast for every single spell in the game)
These are incredibly strong, even on a level 9 mage (he may simply rain down his 4 magic missile and 3 fireball instantly), which is what led the creation of that topic:
Is there anything else to know about Daer'Ragh? He could potentially be the most powerful mage in Faerun if he got another 10-12 level simply thanks to these spells (if he is the only one able to cast them). So why do we not hear of him even a little more, considering he is from Nashkel which is quite close to Amn. He was so cryptical in BG1 and his story was not pushed further in BG2, that s really frustrating