Who were the horadrim?

Question

I now know that the nephalem are the descendants of angels and demons, but how do the horadrim fit in the scene here?

What were they? And why are they seemingly a thing of the past?

Answer 1

The Horadrim were an ancient order of human Mages made by Tyrael to capture the Prime Evils Diablo, Mephisto and Baal in the Soulstones. After they capture them all the threat diminished and they disappeared until only Deckard Cain was left.

http://diablo.wikia.com/wiki/Horadrim

Answer 2

The Horadrim were an ancient order who were tasked with capturing the three prime evils after the Dark Exile (when the lesser evils kicked the prime evils out of hell). Tyrael gifted three soulstones crafted from the worldstone to capture the prime evil's spirits. They were led by Jered Cain and Tal Rasha.

The Members:

The Horadrim were successful as described in "The Binding of the Three":

Eventually, a secretive order of mortal magi was gathered together by the enigmatic Arch-Angel Tyrael. These sorcerers were to hunt the Three Evils and put an end to their vicious rampage. The order, known as the Horadrim, consisted of wizards from the diverse and numerous Mage Clans of the East. Employing disparate magical practices and disciplines....

The first members of the Horadrim came from the remains of the mage clans that had survived the Mage Clan Wars, and were few in number. Estimates put them around seven or twelve in total, though each such mage likely had several retainers, apprentices and other followers with them. These mages were given three soulstones by Tyrael within which they could trap the Prime Evils. (Source)

Their Quest:

They were able to capture Mephisto in Kehjistan and imprisoned him beneath the Temple of Light (Act 3 of Diablo 2). However, when they attempted to capture Baal the soulstone was damaged by Baal. Tal Rasha volunteered to contain the demon inside his own body in combination with the damaged soulstone. This left Jered Cain as the only leader of the Horadrim, but two of the prime evils locked away.

Jered Cain and the remaining Horadrim tracked Diablo further west to Khanduras, following the wake of terror that the demon spread as he went. After an epic battle, Diablo was imprisoned within the remaining Soulstone, and this was placed at the bottom of a large labyrinth within a cathedral in the town of Tristram.

Which is where the huge cathedral central to the Diablo series comes from.

Afterwards:

After Diablo was defeated the Horadrim were left without a major goal besides stopping the prime evils from being unleashed. We learn in Diablo 3 there were some conflicts within the group as Zoltun Kulle alludes to in Act 2.

Following the capture of Diablo and the end of the Horadrim's hunt, Zoltun Kulle returned to Kehjistan where he continued to study magic in great secret. Eventually the rest of the Horadrim learned that he was attempting to create a Black Soulstone with which he would be able to harness the powers of both angels and demons.3 Upon hearing this, the other Horadrim deemed it necessary to kill Zoltun Kulle, but he was so strong that they could not complete the deed. Instead they cut him in half. They removed his head and buried it underground surrounded by magical runes. His body was sent into another dimension.

In the Book of Cain, the author mentions he has heard of no other rebellious Horadrim. He does note that the Horadrim never really recovered from having to hunt down and kill one of their own.

The Book of Cain goes into a bit more detail for the Horadrim after the quest to capture the prime evils. Some returned home and some continued to carry out missions in which they sought to eradicate evil forces. Regardless the order transitioned from being a diverse band of warrior mages to a society of stoic academics. As time passed this resulted in the order's numbers greatly lessening. Cain conjectures this is due to the highly secretive nature of the Horadrim. Even Cain who was part of a long line of the Horadrim had come to doubt the stories by the events of Diablo, so it isn't that surprising.

I guess it is hard to recruit for a secret society of academics to protect against an ancient myth.