The Hundred Changelings were a group of one hundred newly-formed Changelings that the Founders sent out in order to gather information and explore. The Founders did not expect any to return until the late-27th century. (DS9: "The Search, Part II", "Chimera")
In Worlds of Star Trek: Deep Space Nine, Volume 3: The Dominion, Laas returned two more of the Hundred to the Great Link. Odo discovered that the Hundred Changelings were actually sent in an exact pattern, in order to attract the attention of their creator, a giant Changeling. However, it was found and destroyed, presumably by an Ascendant, leading to the disbanding of the Great Link.