Thulsa Doom first appeared in the Kull stories by Robert E. Howard as a powerful undead wizard and necromancer. He declared himself to be the greatest of all wizards and the eternal foe of Kull. He appeared as a man with a skull for a head and eyesockets that burned with unholy flames. He could not be harmed by ordinary mortal weapons, and his stated magical powers included illusion, invisibility, and dimensional travel, though presumably there were many others. He was a follower of the Great Serpent and an ally of the Serpent-Men.
Thulsa Doom unfortunately featured in a single story only ("The Cat and the Skull"), and even then relatively briefly. In that story, he tricked Kull into entering the Forbidden Lake in an attempt to destroy him, but Kull survived and Thulsa Doom escaped. He was euphemistically referred to as the Skull-Faced One.
Conan the Barbarian
In the 1982 film, Conan the Barbarian, Thulsa Doom (portrayed by James Earl Jones) is the enacting lead antagonist. He is a Stygian whose power as a strong and renowned wizard places him as the leader of the unnamed snake cult that grows to be more powerful than most aspiring cults of the same sort.
The cult conducted a series of raids on villages and towns at one point in time before they came to such power, claiming to be in the search for "steel". In the prologue of the movie, one of these raids is seen taking place in the Cimmerian village where a youthful Conan and his parents were living. During the raid, Conan's people, including his mother and father were slain by Thulsa Doom's forces. Conan and the other surviving children were taken to the Wheel of Pain where they were sold into their new lives of slavery. With the given events, Conan seeks out his prioritized revenge against Thulsa Doom, with his only lead being a memory of the symbol of the snake cult.
Thulsa Doom is first notified of Conan taking action when he is informed that Conan has broken in to the Tower of the Serpent and stolen the Eye of the Serpent, along with killing Thulsa's giant serpent pet that was defending it. One of Thulsa's lieutenants, Rexor, threatened King Osric after the incident, and in anger the cult seduced the King's daughter to travel to the Mountain of Power temple to meet Thulsa and pledge herself as his slave.
Thulsa's second meeting with Conan takes place after Conan's failed attempt to sneak into the Mountain of Power temple. After some philosophical speech, and acknowledgement of the pains that both Conan and Thulsa have inflicted on each other, Thulsa orders Conan to be crucified on the Tree of Woe, thinking that it would be the last time he saw him.
Another scene follows later in the film where Conan carries out a second attempt to invade the Mountain of Power temple, this time with his companion Subotai and his lover Valeria. As the three are silently executing the guards, Thulsa Doom begins a transformation while he sits upon his throne, and his human features begin to morph into those of a serpent until he has become a large snake and slithers out of his robes and into a small transport tunnel which no one could follow him due to the size being only able to fit a snake, not a human. This robs Conan the chance of revenge, and so the three only kidnap King Osric's daughter, who is unwilling to leave Thulsa. All four escape on horseback, but not before Thulsa reappeared at the back entrance of the mountain in his human form, where he reveals a snake in his hands, whispering to the small serpent "seek". He straightens out the snake and draws his bow, after which he fires the snake like an arrow and it pierces Valeria in the side as they rode away, killing her soon following.
Thulsa and his two lieutenants, Rexor and Thorgrim, plan to carry out an effort to retrieve the princess and return her to the temple. Meanwhile, Conan, Subotai, and the Wizard have set up traps and ambush spots for their oncoming enemies in the place of the sacred standing stones, knowing that they will be heavily outnumbered. Thulsa over watches the battle, and sees all of his men and both lieutenants slain by the three men. He decides that if he cannot have the princess, nobody will, and prepares another enchanted snake arrow to kill her despite the girl's desperate pleas. Subotai, however, races up the hill and is able to block the arrow with his shield at the last moment. Doom rides off, and the princess now appears to be free of his indoctrination.
The final scene of the movie shows a night ceremony at the temple on the Mountain of Power where Thulsa is addressing his followers. Conan and the princess move throughout the temple, her being the distraction to the guards while Conan eliminates them one by one. They make it to the top of the temple where Doom stands, now without any bodyguards. Conan stands next to him, ready to execute him, but Thulsa's enchanting voice takes power and begins to convince Conan to reconsider his action. Conan slowly realizes that he is being hypnotized the same way his mother was before Thulsa killed her, and he breaks out of the trance to slash at Doom's neck. Thulsa begins to bleed out and falls to his knees at the top of his temple, facing the audience, and Conan decapitates Thulsa in front of the cult, leaving his body to fall from the edge, and holding Doom's head high in the air for all to see before throwing it down the stairs. The followers begin to disperse, and Conan ignites the temple, leaving with the princess. Thulsa and his snake cult are now both destroyed.
"Yes! You know what it is, don't you boy? Shall I tell you? It's the least I can do. Steel isn't strong, boy, flesh is stronger! Look around you. There, on the rocks; a beautiful girl. Come to me, my child...
[coaxes the girl to jump to her death]
That is strength, boy! That is power! What is steel compared to the hand that wields it? Look at the strength in your body, the desire in your heart, I gave you this! Such a waste. Contemplate this on the tree of woe. Crucify him!" - During Conan's capture at the Mountain of Power temple (01:14:31)
"Purging is at last at hand. Day of Doom is here. All that is evil, all their allies; your parents, your leaders, those who would call themselves your judges; those who have lied and corrupted the Earth, they shall all be cleansed." - Thulsa Doom's final speech to his followers in the director's cut (01:57:31)
"My child, you have come to me my son. For who now is your father if it is not me? I am the well spring, from which you flow. When I am gone, you will have never been. What would your world be, without me? My son." - Last words to Conan (01:59:05)