For the zone, see Dahlgur Oasis.
The Dahlgur Oasis is a tome found in Act II of Diablo III, inside a Keepsake Box in the Dahlgur Oasis, usually right next to the small pond east from entrance.
This tome is required for the Historian of Caldeum achievement.
It reads as follows:
Three centuries ago, a much smaller Caldeum depended upon small wells for its water. Then came the mysterious Dahlgur, offering Caldeum an oasis of unparalleled splendor located in an area previously thought to be desert, asking only that his name be permanently affixed to the land. Then, he vanished into the wastes. — Abd al-Hazir