The Order of the Liches is possibly the oldest order of all, founded mere years after the creation of the Altars of Eximagdra. It is always led by a Lich King, or Queen.
As the name says, the order exists out of only Liches, well, mostly at least. Imrboss is the creator of Undead as a whole, including making the first Liches, some of which are still alive and part of this order. Besides here two Divine Ones are also members.
Imbross is one of the Platinum Dragons, and creator of the Undead, and thus the Liches. When Imbross wasn't yet imprisoned, she ruled over the order.
See: Imbross, Order of the Liches
Zul'Nec-ro is a Divine One with dominion over death and lightning.
Negos leads the Dark Riders, who are the Liches' elite army.
See: Negos, Dark Riders, Order of the Liches
The very first Lich King, one of the original Liches, king of the old Dwarven kingdom of Steelmountain.
Arathor was the second Lich King, crowned by Imbross. Arathor used to be the Human crown prince of Stormhaven. About 4000 years ago he was defeated in combat, and his crown was taken by Nerdril.
The current Lich King, after Arathor. He keeps the Order of the Liches running. Nerdril is a Void Elf, and has been the Lich King for about 4000 years.
Morgon was one of the original Liches, like Thargun. Since the very beginning he has served as second in command to the Lich King, and often as the leader of the army.
A Shadow Elf Lich, ranger general of the Lich King. Kaela has been around for quite a while now, though not as long as Morgon or Nerdril.
Rulthad was once a noble High Elf, and a powerful necromancer. He used to be a Sunstrider, but was cast out. This led him to seek out the Lich King, which was Arathor at that time, and become a Lich himself. Since the last 4000 years he has served him as the guardian of the throne.
Kal'Thozul was a Troll before he became a Lich. Once upon a time, Kal'Thozul led a rebellion against the Lich King, and after some time he was slain. In his rebellion, he started the Cult of the Damned, which he led for a while.
She used to be a Foxfolk slave, but this she accepted no more. She prayed to Vulpes for strength, and she gave it. Vulpes led Daku to Zul'Nec-ro. He then turned her into a Lich. Daku is now an emissary of the Order of the Liches in the Plains of Ruthal, watching over the Cult of Sethraliss.
Bobobo is the newest of the Liches. Being personally protected by Imbross, though even that didn't stop him from dying permanently.
See: Bobobo, Owlfolk, Imbross, Bobobo's Army, Cult of the Damned, Heroes of Goldcrest, Order of the Liches