After the Wyvern Massacre, Demona roamed Scotland alone for a time, finding food wherever she could, including human farms and food stores. Over time she encountered other rogue gargoyles and pulled them together to form a new clan.  Some surviving members of the Wyvern splinter clan also came to join.  She quickly asserted herself as leader of the clan and appointed a second-in-command, though she did not give him much authority. 
By 997, Demona had the clan spending their days in separate cells around Scotland in an attempt to protect against another clan-wide massacre. This was wise as Constantine had initiated a gargoyle genocide throughout Scotland and was aided by none other than Gillecomgain who would grow to be the first Hunter. Together, they destroyed the Sruighlea Cell, much to the shock and dismay of the surviving cells. Shortly thereafter, Brooklyn, who had come to this time as part of his TimeDancing, sought out Demona and asked for her clan's help. Demona called all her cells together to join the Battle of Rathveramoen against Constantine, with the intent of turning on the victor of the battle and taking control of Scotland for the gargoyle race. It is unknown what happened to the clan immediately following the battle's end. ("Tyrants," "Phoenix")
By 1020, Demona's clan was still quite small, and still lived in several cells. Her main cell, the Lunfanan Cell, consisted of six gargoyles, she being the only female. By this time, it seems that all the gargoyle beasts had died or been killed. Her clan inhabited caves and beyond those shelters, it seems unlikely they had any real protectorate, their main focus being mere survival. Demona led her clan in raids against humans for food, much to the dismay of at least some of her clan. ("City of Stone Part Two")
The small clan continued its struggle for survival unscathed for another two decades, but in 1040, Duncan and Macbeth discovered the Lunfanan Hill Cell asleep in a cave. Duncan prepared to destroy them in their stone sleep, but Macbeth begged him to spare them due to his history with Demona. Duncan seemed to agree, but later that day returned to the cave with some of his men and managed to destroy at least one clan member shortly before they woke at sunset. The band of gargoyles battled their way free from the cave and went on to find a new lair.
Shortly thereafter, Macbeth negotiated an alliance with Demona, finally bringing a real truce between Demona's clan and a group of humans, in this case, Clan Moray. Demona's clan fought in a major battle against Duncan's forces, proving crucial to Duncan's defeat. After the battle, Demona and her clan were praised for their help and after Macbeth was crowned King of Scotland, he assigned Demona his primary advisor, hoping to usher in a new golden age between gargoyles and humans. ("City of Stone Part Three")
For a time, Macbeth's hopes seemed to come to pass. Demona's clan survived at or near Castle Moray for seventeen years. The clan remained small, but was successful and, by 1057, they numbered around two dozen. Demona continued to divide them up into cells to insure that if one group was destroyed, the others would survive. When Canmore and his English allies attacked Scotland that year, Demona's clan fought against them, holding them back. Sadly, Demona, in her paranoia, felt she and her clan were going to be betrayed by Macbeth and formed a deal with Canmore to betray Macbeth first. Canmore in turn betrayed Demona, and destroyed her entire clan, leaving her, once again, the lone survivor of the massacre of her clan. ("City of Stone Part Four") As Gruoch had ordered her, Demona searched for her kind throughout Scotland, looking for cell after cell, but found none alive. It is unclear how Canmore managed to destroy the entire clan in a short time despite their being divided up. It is possible that because they had grouped together as an army and/or they were deserting their usual hidden lairs they were easily tracked and destroyed.
Though no rookery or eggs were ever shown in connection with Demona's clan, it is possible a rookery could have existed as the need arose.  There may have been rookeries hatched in 998, 1018, and 1038 and eggs due to hatch in 1058. In any event, if there were any young gargoyles and/or eggs, Canmore probably had them destroyed as well. 
Demona once remarked that the Hunter destroyed the last gargoyle clan in Scotland. Though she was clearly incorrect because the Loch Ness Clan survived to the present, her statement is probably referring to her clan. Greg Weisman has said that following the clan's destruction in 1057, Demona did not encounter any other gargoyle clans in Scotland at least. This also seems to indicate her clan was the last gargoyle clan destroyed in Scotland, though it wasn't the last one to exist there. The clan's alliance with Macbeth and his people represents what was probably the last human-gargoyle partnership in Scotland, and probably all of Europe. Despite Macbeth's hopeful words at his coronation, the alliance between Demona's Clan and Clan Moray didn't just mark the beginning of a golden age of human-gargoyle relations, but also the end of this age, in that region and time at least.
As far as was seen in the series, the members of this clan shared the same physical characteristics as those of the Wyvern Clan and Avalon Clan including a variety of leathery wings, different shapes, sizes and number of brow horns, diverse coloration in skin and hair, and other features shown in gargoyles of their race. However, since Demona's Clan was a conglomeration of rogue gargoyles from numerous destroyed clans, there may have been gargoyles among the clan of different racial backgrounds, including members similar to those gargoyles found in London or Loch Ness. According to Greg Weisman, Demona was "pretty much" the only named gargoyle among her clan. 
Though in the early years of this clan, at least some beasts were known members (one beast in the Sruighlea Cell and two in the Wyvern Cell), a few decades later none were seen to exist. It is possible their small numbers doomed them to simply die out or perhaps other factors may have contributed to their demise.
Interestingly, the five members of the Wyvern Cell in 1020 (see picture above) had very similiar coloring to Goliath, Hudson, and the Trio though most other features were dramatically different. This is probably coincidence.
- Blue Beaked Gargoyle
- Seven gargoyles
- One beast