Pre-Series
Lord Darkar was formed at the beginning of time, when all things were created, along with his polar opposite, the Great Dragon.
At some point, the Ancestresses became his servants, though it is unknown how and when this happened. Given that the Ancestral Witches have been called the "First Creatures of Evil" and that Darkar himself is Evil personified, there might be a possibility that he was responsible for the creation of the three ancient Witches.
The three witches found a spark of the Dragon Flame which had mixed with the darkness, and shaped it into a man they named Valtor, brought him up and sent him to conquer the Magic Dimension. Given that Lord Darkar is darkness personified, it is possible that this dark spark consisted of the power of the Great Dragon mixed with that of the Shadow Phoenix.
Some time before the destruction of Domino, Lord Darkar tasked the Ancestral Witches to acquire the Dragon Flame, which he needed to obtain the Ultimate Power of the Relix Dimension, and King Oritel and Queen Marion of Domino formed the Company of Light, a group of magical warriors, to fight against the Witches.
The Witches therefore attacked Domino to obtain the power of the Dragon, but the Princess Daphne was able to prevent this from happening by sending the current keeper of the Dragon's Flame, her baby sister Bloom, to a distant place where the Witches would never find her. The Witches were themselves defeated by the Company of Light and imprisoned in the Realm of Obsidian while Valtor was imprisoned on the Omega Dimension. However all the inhabitants of Domino, including King Oritel and Queen Marion, became trapped on Obsidian and all of planet Domino became frozen during the final battle during which the Company of Light defeated the Ancestral Witches.
When the Witches were defeated by the Company of Light, the Lord of the Templar of the Fortress of Light fought Darkar himself, cast him back into the darkness, and sent him into a deep slumber for sixteen years, until he woke up again, possibly caused by the return of Bloom's Dragon's Flame in the Magic Dimension after she used it to defeat the Trix in the first season, since his power is opposite to that of the Dragon's.
This also means that Valtor was also serving the Shadow Phoenix at this time, meaning that Lord Darkar already had someone with the Dragon Flame serving him, and the only explanation that can be given to his causing the destruction of Domino is that Valtor's Dragon Flame was the Dark part of the flame which was created when a spark from the Dragon's fiery breath had mixed with the Darkness (possibly itself having being created from Lord Darkar's power, since it was stated that both Lord Darkar and the Great dragon were born at the beginning of Time from the Void), and therefore was halfway between the powers of Light and those of Darkness. Darkar needed a power which was the complete opposite to his own power to create a balance between Light and Darkness in order to summon and acquire the Ultimate Power from the Realm of Relix, and the pure Dragon Fire of the Royal Family of Domino was what he needed.
It can also be supposed that Mandragora was also serving Darkar, since she was herself the servant of the Ancestresses who were themselves Darkar's servants. Moreover, from the reactions of members of the Company of Light in the first movie it can be guessed that they had already fought Mandragora when the Company had recently been formed, around the time of the destruction of Domino and hence that she had probably taken part in the destruction of Domino, which was done under Darkar's orders.
Series
Season 2
Darkar's ambition is to capture the Ultimate Power of the Relix. However, the portal to the Relix realm can only be opened by the four pieces of the Codex, however there is one in each of the schools; Red Fountain, Alfea, Cloud Tower, and in Pixie Village. He holds the pixies hostage to get them to reveal the location of the Pixie Village, but they refused to do so. The princess of Andros, Aisha, despite her best efforts, could not save them. She then got the Winx's help when they rescued her. Unfortunately, Darkar also got help from the Trix. He did this by releasing the Trix from the Fortress of Light (Roccaluce) to assist him in his quest and gave them Gloomix power.
After he obtained two of the four pieces, he made the fake Avalon that he had sent to Alfea to put a shadow virus on Bloom to turn her into Dark Bloom. Darkar then used her as his puppet to get Alfea's piece of the Codex. He obtained the final piece of the Codex when he sent Icy after Livy to get Pixie Village's part of the Codex. Icy succeeded with the help of her sisters, Darcy and Stormy.
He then ordered Avalon's shadow clone to abduct Bloom and when she was in his castle, he cast the Shadow Spell on Bloom again and used her and her Dragon Flame, since he needed a power of Light to counter his power of Darkness, to open the Portal to Relix. But when the Trix confronted him, saying they did all the work, he told them that they had only been his pawns and were now useless to him and sent them to the Dark Void: Oblivion. He almost succeeded in gaining the Ultimate Power of Relix, but as he was absorbing the power, Sky told Bloom that he loved her and the love for him in her heart caused her to use her healing power on herself and the others and broke the Shadow Spell as well as destroying the Power.
The Trix, fused into one body, tapping into their Gloomix, took on Darkar, but were knocked out. All six of the Winx members used a Charmix Convergence to defeat Darkar and, according to Faragonda, destroyed him. The Trix (still fused into one body) remained imprisoned in Relix, which was then apparently sealed for good since the Codex and its copy had been used and no other Codex has been shown to exist, till they were sent to the Omega Dimension some weeks later (it is not known how the authorities of the Magix were able to go to Relix to catch the Trix since the only known pieces of the Codex had all been used). His Castle collapsed after he was destroyed, probably because it was his Evil powers which were maintained in it. It is likely all his shadow monsters were destroyed as well.
Season 3
He was mentioned in a few episodes. First in "The Heart and the Sword", Vanessa tells Bloom when Darkar had placed her under a dark spell and Sky confessed his love for Bloom to save her.
Then in "Alfea Under Siege" when Stormy suggests to Valtor about setting up quarters at Darkar's Fortress when he was not satisfied with his hideout in Andros anymore, but it was rejected by Darcy.
And lastly, in "Wizard's Anger" when Icy complains to Darcy and Stormy about how they get abused by Darkar and then Valtor.
Season 4
In the first episode of the season, a portrait of Lord Darkar is seen alongside one of the Trix, Valtor and the Wizards of the Black Circle within a section of the Hall of Enchantments reserved for keeping record of all the Magic Dimension's greatest threats.
In the second episode of the season, Darkar appears in a flashback where he, in the form of Cobra, scares Pixie.
Specials
The Shadow Phoenix
Darkar has kidnapped the Pixies from their village and tortured them for the location of the Codex. He had also captured Aisha who escaped but returned to rescue her Pixie friends. Darkar is aware of this and sends his minions to recapture them all but when they fail, Darkar takes the matter into his own hands.
He confronts a worn out Aisha at the entrance of his domain and asks Aisha to return the Pixies to him. When she refuses he forcibly strikes her and take back the Pixies. He finds it foolish when Aisha fights back, who claims that "a fairy never gives up without a fight!", bemused he levitates her over the edge of his castle and drops her.
Once he re-imprisons the Pixies, he locates the Trix, free them from their imprisonment at Lightrock Monastery so that they can serve as his servants. He gives them their Gloomix powers and orders their assists in retrieving the Codex from Pixie Village. He decides to first use the Winx to help him and spies on them again; he learns that Bloom, Stella and Aisha will try to sneak into his domain again to rescue the Pixies. He sends the Trix to greet them.
Once the Trix had done their part, he swoops in to "save" the fairies from the Trix disguised as a paladin. He bows to them and flies away, leaving the fairies impressed. And in a timely manner, he sets the Pixies free as well.
The next day, he is at Alfea and is introduced by Faragonda to the Winx as their newest faculty staff, Professor Avalon. While there, he gains their trust, allowing him to carry his plan without raising suspicion. He then uses one of the potion rooms to secretly create a homesickness spell and place it on the Pixies (who were unable to return home as they knew it would lead him to their village). The spell worked and Darkar sends Icy to follow the Pixies and succeeds in retrieving the Codex.
His next agenda: kidnap Bloom. As she is the Keeper of the Dragon's Flame, she is the key to unlocking the Ultimate Power found in the Crystal of Power. As Avalon, he tricks her to enter a trap bubble. With her in it, it teleports them to his domain where he turns her into Dark Bloom. After he opens the portal to Relix, the Trix confront him for using them to distract Bloom's friends and not giving them their share of the ultimate power. He is amused that they trusted him and sends them back to their prison. He then enters Relix with Dark Bloom and while she chants the incantation, they are interrupted by Stella, Aisha, Tecna, Musa and Flora.
Dark Bloom has to restart the incantation chant and while she does so, Darkar deals with her friends and then the Specialists. He suspends their movement and after the Crystal is summoned he transforms into his phoenix form and begins absorbing its power. He calls Sky a fool when he tries to reach Bloom by confessing his feelings for her. When Sky faints, Darkar laughs and continues absorbing the power until he realizes that Bloom is returning normal. He is prevented from absorbing any more power once Bloom returns the Crystal to the Relix.
Angered, he strikes them but is blocked by Griffin and Faragonda's barrier while the Winx converge their powers and destroy him once and for all.