I remember this story from sometime in the 90s. It was in an issue from a magazine or comic about classic Conan.
The story is "The Devil in Iron" by Robert E. Howard; text available at Project Gutenberg, audio at LibriVox. The story has appeared in a few comic books; the one you remember from the 90s is probably the reprint in Conan Saga #46, January 1991.
The story that I want to find has Conan finding a cyclopean city and a man with massive muscles sleeping on a stone slab just outside the city. The man has a simple bronze knife on his chest and wears a loincloth. He takes the knife and the man wakes up.
That's pretty much what happens, but it was a fisherman who took the knife and woke the sleeper before Conan arrived on the scene:
Among the trees reared a broken dome-like structure, built of gigantic blocks of the peculiar iron-like green stone found only on the islands of Vilayet. It seemed incredible that human hands could have shaped and placed them, and certainly it was beyond human power to have overthrown the structure they formed. But the thunderbolt had splintered the ton-heavy blocks like so much glass, reduced others to green dust, and ripped away the whole arch of the dome.
The fisherman climbed over the debris and peered in, and what he saw brought a grunt from him. Within the ruined dome, surrounded by stone-dust and bits of broken masonry, lay a man on the golden block. He was clad in a sort of skirt and a shagreen girdle. His black hair, which fell in a square mane to his massive shoulders, was confined about his temples by a narrow gold band. On his bare, muscular breast lay a curious dagger with a jeweled pommel, shagreen-bound hilt, and a broad crescent blade. It was much like the knife the fisherman wore at his hip, but it lacked the serrated edge, and was made with infinitely greater skill.
The fisherman lusted for the weapon. The man, of course, was dead; had been dead for many centuries. This dome was his tomb. The fisherman did not wonder by what art the ancients had preserved the body in such a vivid likeness of life, which kept the muscular limbs full and unshrunken, the dark flesh vital. The dull brain of the Yuetshi had room only for his desire for the knife with its delicate waving lines along the dully gleaming blade.
Scrambling down into the dome, he lifted the weapon from the man's breast. And as he did so, a strange and terrible thing came to pass. The muscular dark hands knotted convulsively, the lids flared open, revealing great dark magnetic eyes whose stare struck the startled fisherman like a physical blow. He recoiled, dropping the jeweled dagger in his perturbation. The man on the dais heaved up to a sitting position, and the fisherman gaped at the full extent of his size, thus revealed. His narrowed eyes held the Yuetshi and in those slitted orbs he read neither friendliness nor gratitude; he saw only a fire as alien and hostile as that which burns in the eyes of a tiger.
The man possibly tells him about how he is actually a primordial being that first had the shape of a creature of black slime. Eventually he became the form he had, and taught early man architecture. They worshiped him as a god.
Conan overhears Khosatral talking to himself, and has a vision:
He found himself in a chamber, not another corridor, and was about to retrace his steps, when he heard a voice which came from behind one of the walls. There was no door in that wall, but he leaned close and heard distinctly. And an icy chill crawled slowly along his spine. The tongue was Nemedian, but the voice was not human. There was a terrible resonance about it, like a bell tolling at midnight.
'There was no life in the Abyss, save that which was incorporated in me,' it tolled. 'Nor was there light, nor motion, nor any sound. Only the urge behind and beyond life guided and impelled me on my upward journey, blind, insensate, inexorable. Through ages upon ages, and the changeless strata of darkness I climbed—'
Ensorcelled by that belling resonance, Conan crouched forgetful of all else, until its hypnotic power caused a strange replacement of faculties and perception, and sound created the illusion of sight. Conan was no longer aware of the voice, save as far-off rhythmical waves of sound. Transported beyond his age and his own individuality, he was seeing the transmutation of the being men called Khosatral Khel which crawled up from Night and the Abyss ages ago to clothe itself in the substance of the material universe.
But human flesh was too frail, too paltry to hold the terrific essence that was Khosatral Khel. So he stood up in the shape and aspect of a man, but his flesh was not flesh, nor the bone, bone, nor blood, blood. He became a blasphemy against all nature, for he caused to live and think and act a basic substance that before had never known the pulse and stir of animate being.
He stalked through the world like a god, for no earthly weapon could harm him, and to him a century was like an hour. In his wanderings he came upon a primitive people inhabiting the island of Dagonia, and it pleased him to give this race culture and civilization, and by his aid they built the city of Dagon and they abode there and worshipped him. Strange and grisly were his servants, called from the dark corners of the planet where grim survivals of forgotten ages yet lurked. His house in Dagon was connected with every other house by tunnels through which his shaven-headed priests bore victims for the sacrifice.
Eventually he was put to sleep with the knife on his chest.
But after many ages a fierce and brutish people appeared on the shores of the sea. They called themselves Yuetshi, and after a fierce battle they were defeated and enslaved, and for nearly a generation they died on the altars of Khosatral.
His sorcery kept them in bonds. Then their priest, a strange gaunt man of unknown race, plunged into the wilderness, and when he returned he bore a knife that was of no earthly substance. It was forged of a meteor which flashed through the sky like a flaming arrow and fell in a far valley. The slaves rose. Their saw-edged crescents cut down the men of Dagon like sheep, and against that unearthly knife the magic of Khosatral was impotent. While carnage and slaughter bellowed through the red smoke that choked the streets, the grimmest act of that grim drama was played in the cryptic dome behind the great daised chamber with its copper throne and its walls mottled like the skin of serpents.
From that dome the Yuetshi priest emerged alone. He had not slain his foe, because he wished to hold the threat of his loosing over the heads of his own rebellious subjects. He had left Khosatral lying upon the golden dais with the mystic knife across his breast for a spell to hold him senseless and inanimate until doomsday.
The man somehow menaces him and Conan instinctively stabs him with the knife. The creature progressively returns to the shape of the primordial slime and dies.
Then Khosatral turned again, flailing the air with desperate blows, but Conan, fired to berserk fury, was not to be denied. As a panther strikes down a bull moose at bay, so he plunged under the bludgeoning arms and drove the crescent blade to the hilt under the spot where a human's heart would be.
Khosatral reeled and fell. In the shape of a man he reeled, but it was not the shape of a man that struck the loam. Where there had been the likeness of a human face, there was no face at all, and the metal limbs melted and changed . . . Conan, who had not shrunk from Khosatral living, recoiled blenching from Khosatral dead, for he had witnessed an awful transformation; in his dying throes Khosatral Khel had become again the thing that had crawled up from the Abyss millenniums gone. Gagging with intolerable repugnance, Conan turned to flee the sight; and he was suddenly aware that the pinnacles of Dagon no longer glimmered through the trees. They had faded like smoke—the battlements, the crenellated towers, the great bronze gates, the velvets, the gold, the ivory, and the dark-haired women, and the men with their shaven skulls. With the passing of the inhuman intellect which had given them rebirth, they had faded back into the dust which they had been for ages uncounted. Only the stumps of broken columns rose above crumbling walls and shattered dome. Conan again looked upon the ruins of Xapur as he remembered them.