The Seekers of Truth in Kirkwall secretly created the Band of Three to investigate why where are so many blood mages in the city, the nature of the Veil there and whether the Forgotten Ones in elven lore are connected to the Forbidden Ones in other works. The team, comprised of a Dalish apostate Vahnel, Brother Kerowen, and Felestia, was bound with holy vows to uncover what lore they could and return it to the Seekers. 
Hawke can find notes detailing their discoveries. Hawke can discover more passages as time passes in Kirkwall. The notes explain how the Band of Three conducted their research and include some comments on their theories.
The Band of Three disagreed with the then-accepted theory that the Tevinter Imperium came to Emerius (future Kirkwall), for its natural harbor and mineral wealth. They also note the Imperium dispatched three legions to Emerius which never arrived and seemingly vanished. Suspecting a greater reason, they continued their research, delving into continually more dangerous situations.
Upon acquiring some rare texts from Kirkwall's black market, the Band realized the city had hidden undercity passages which had housed Tevinter researchers. As they explored the passages, they wondered why Kirkwall's magical research was hidden, when so many other cities in the Imperium studied it publicly. Eventually, a stonemason showed the Band a map of Kirkwall, and the writer saw that the city had been built according to magical symbols; this, apparently, is why Kirkwall is very easy to get lost in.
During the Tevinter Imperium in Kirkwall, a large number of slaves would disappear every year. Though Kirkwall was a slave-trading hub and required large numbers of slaves in order to work the mines, the Band notes that roughly one of every ten slaves went missing, a disproportionately high number.
One of the notes mentions that the Band was eventually attacked by maleficarum, resulting in Vahnel being severely injured and perhaps his or her death. Interestingly, they note that Kirkwall's Circle of Magi has consistently had problems with blood mages and Harrowing deaths ("almost double that of Starkhaven or Ostwick"), something consistent with Hawke's experiences in Kirkwall.
During their research, the Band found some disturbing records about Kirkwall's architecture. The city's sewers have grooves that lead downward, apparently for blood sacrifice. They note that blood magic from a simple cut is already very powerful, and could not understand why the Imperium needed blood magic on the scale of thousands of slaves a year.
Eventually, the Band's research culminates in the discovery that the Imperium had been deliberately weakening Kirkwall's already thin Veil, though they do not know for what purpose. It can be inferred from the notes that Kirkwall's design was intended to be used as part of a blood ritual to do this, perhaps in order to summon demons like the Forbidden Ones.
The final note left by the Band suggests that they may have attempted to recreate the magister's lore they gathered. Whatever they did killed Felestia and left the writer severely injured. The last member of the Band, Kerowen, returned to seal whatever they released, saying the knowledge they had gathered must be destroyed.
- The demon Xebenkeck from the quest Forbidden Knowledge, one of the Forbidden Ones who taught humans how to use blood magic, guards the Fell Grimoire mentioned in the notes. But the last note calls her "the Forgotten One" - one of the evil elven gods who were cast into the Abyss.