They are an enigma to the inhabitants of Tamriel, and only those who make deals with them or have visited the Soul Cairn know of their existence. Most individuals who deal with the Masters are deceived, and subsequently their souls are stolen. While there are theories, exactly why the Ideal Masters crave souls is unknown.
It has been speculated that the Ideal Masters have transcended beyond physical manifestation and now exist as non-corporeal beings. When they do interact with others, be it to feed on their victims or speak to their underlings, they do so through the crystalline structures that dot the Soul Cairn.
The Ideal Masters' weakness is their hunger for pure souls. This hunger is believed to be the reason for the existence for the Soul Cairn itself, and it is often the only leverage necromancers have when bargaining with them. In return, the Ideal Masters grant them the ability to conjure powerful undead guardians in a similar manner that conjurers would summon an Atronach or Daedra.
In An Elder Scrolls Legend: Battlespire, the player will speak to the Ideal Masters as part of the Soul Cairn questline. Since the Daedric armies of Mehrunes Dagon invaded the Soul Cairn, the Ideal Masters needed assistance in ridding the Soul Cairn of the Daedra. The Daedra planned to use it as a waystation between their own realms and their newly-conquered domains. The Daedra killed the Masters' servants and plundered their treasure, although they also released the souls of several prominent mortal allies whom Dagon had betrayed into the realm. An apprentice battlemage from the Battlespire entered the realm, following the retreating army. Most of the Ideal Masters are unwilling to help the player, but when they learned of the Daedric invaders who would supposedly continue to regularly pass through their realm, one Master instructs the player on how to use the Emerald Gates to leave the realm and continue stalking Dagon's army. The gates can then be used to travel to the Shade Perilous.
At some point in history, the masters asked Durnehviir to guard the Soul Cairn until a powerful necromancer named Valerica died. Durnehviir, not knowing that she was a vampire and therefore immortal, accepted and was doomed to guard the Soul Cairn for eternity.