Doctor Mo was not suffered to rejoice many days; the might of the soul-oath blood curse showed itself before long, and he began to age with a horrifying speed nigh to one year for every day that passed, growing old almost before one's eyes.
He was seized with a terrible dread, and racked his wits for a way to master the strange thing that stalked his body; but whatever he tried availed him little.
If matters were suffered to go on thus, he would perforce, before long, breathe his last, decaying into death like any aged man. But, to his good fortune, Yu Zitong's primordial spirit was yet worse off.
It came to this: once Yu Zitong had entered Doctor Mo's body, as the time wore on there grew upon him a strange thing — the very assimilation of his spirit by his host.
This "assimilation" was a passive form of soul-possession. When a wandering spirit abides overlong in another man's body, it is gradually bent and moulded, all unaware, by the incumbent soul of that body, the two working upon each other, until in the end but one mind may be left alive within it — a most baleful and savage affair.
Seeing how the matter stood, Yu Zitong had no recourse but to turn his thoughts to a bold, deliberate soul-possession of his own.
And if he was so loath to do it, it was not from any gentleness of heart, but because he dreaded the Three Iron Laws of soul-possession, of which all the tales of the immortal world speak:
First, a cultivator may not seize upon the body of a mortal; else the body so seized, unable to withstand the act, would crumble and collapse of itself.
Second, only when one of greater power seizes upon one of lesser power can the act hope to succeed without suffering the other's backlash; and the greater the gulf between their powers, the safer the deed.
Third, a cultivator, in all his life, be his power high or low, may work soul-possession but once; upon a second attempt, his primordial spirit will perish without rhyme or reason.
These three iron laws, unbroken by countless attempts, had checked more than one villain who would have used the art to raise a storm, and more than one trickster who would have used it to dodge the fate that dogged him. Heaven, in its warning against such heaven-defying deeds, would not suffer a cultivator to throw all the world into disorder and ruin beyond all mending by means of this art.
And so it was that, had Doctor Mo been a cultivator himself, Yu Zitong would not have been afraid at all; he could have fought the other tooth and nail, and wrangled with him for the possession of the very body. But Doctor Mo was but a mortal, with not the least power to his name; he could not possibly endure an act of soul-possession, and it was to be feared that, even should the seizure but run half its course, the body contested for would collapse into utter ruin.
Nor would it better his case to seek out some other man's body to dwell in; that could not avoid the fate of assimilation again, and would but bring him into his old straits once more, and worse besides. For his power would dwindle sharply with every going out and coming in of his spirit, and soon be spent utterly; he would be trapped alive within some other man's body, and at the last be assimilated.
For when a man is turned into a primordial spirit, he has no body where he might sit in meditation and replenish himself; the power he carries is spent with every use, and fades by degrees with the passing of time. He could not tell how long he might yet hold out.
And so Yu Zitong, unless he could find a cultivator of slight power whose body could yet endure an act of soul-possession, would by no means leave his own spirit again, to gamble upon so rash a hazard.
Pressed upon the one side by the dread that Doctor Mo's body, crumbling under the blood curse, should leave his spirit nowhere to abide, and upon the other by the danger of being assimilated by his host's soul — under both these enormous pressures did Yu Zitong, who clung to life above all things, after much weighing of the matter lay by for a while his old enmity, and, sorely against his will, make contact with Doctor Mo, and set out before him the whole truth of the affair and all the advantages and perils it held.
Doctor Mo, hearing all, was at first somewhat angered; but he was quick to grasp the great fortune the chance offered, and, with no more ado, struck a bond with Yu Zitong under set terms, revealing the very nature of that consummate rogue he was.
First, Doctor Mo was to govern his own mind after the method Yu Zitong should teach him, and do all he might to avoid assimilating the other's spirit. For his part, Yu Zitong would impart to the doctor certain secret arts by which he might slow the pace of his ageing, and hold, for a while, some measure of power.
Second, Doctor Mo was to seek out a lad possessed of a spiritual root, who could cultivate the Art of Eternal Youth; and, having taught him the art, was to await the ripe hour, and then, trusting in the power he had momentarily gained, work his soul-possession and be reborn.
On this point Doctor Mo had once harboured a doubt, and had sought to cultivate the art himself; but he had come to nothing, and had been laughed to scorn by Yu Zitong for his pains. Only then did he learn that one without a spiritual root can never refine true power out of himself — and that he himself was one of those dull folk the cultivators call "without a root."
Third and last, once Doctor Mo, his soul-possession accomplished, had won himself ample respite, he was then to find for Yu Zitong a fitting body, and help him accomplish his own seizure.
These terms, at first blush, leaned rather to Doctor Mo's advantage; but there was no helping it, for the danger of assimilation hung at that very moment over Yu Zitong's head. Standing in so poor a case, he could but give way and suffer some loss — though whether he was in truth the loser, only he himself could say.
Through all this, Yu Zitong had once proposed that Doctor Mo go to his clan's hidden fastness for aid; but Doctor Mo, too wily by far to put a handle into the other's hands, refused the thing without the slightest room for argument — a refusal for which Yu Zitong gnashed his teeth in hatred all the days thereafter.
As for what followed, there was little left to tell. The toils of finding a fit subject had wearied Doctor Mo those first years; disheartened, he had entered the Sect of Seven Mysteries; and there, by chance, he had taken Han Li as his disciple and taught him the Art of Eternal Youth — all of which differed little from the doctor's own telling, being, moreover, a thing that Han Li himself had lived through.
Han Li, when he had heard all this out, let forth a long breath, and the greater part of the tangles in his mind was at last untied.
Yet, seeing Yu Zitong fall silent and make as if to say no more, Han Li darkened and asked, in a cold voice:
"There is one thing you have not yet told me — how it came about that Doctor Mo met his death!"
"There is nothing to explain. Doctor Mo had but grossly misjudged the progress of your Art of Eternal Youth. His power came far short of yours, so that his seizure failed, and, turn and turn about, he was swallowed up by you instead." The tone of Yu Zitong's voice wavered before he gave out the plain truth.
"Then the yellow orb that first entered my body was Doctor Mo's primordial spirit, and the green one that followed was yourself," said Han Li, with an air of great indifference.
"This... this — at the time I had thought that you and Doctor Mo had destroyed each other! Not wishing to waste the body, I thought to make use of it." Somewhat abashed, he spoke.
"Hmph. I do not think it was a mistaken notion. I think it was a trap you laid with deliberation."
"Yu Zitong, when you first taught Doctor Mo the Great Art of Soul-Possession, you had no good intent from the first. You made a point of never mentioning that success or failure hung upon the height of one's power."
"By your original design, Doctor Mo, working the soul-devouring art upon himself and my fourth layer of the Art of Eternal Youth, would have found their powers all but evenly matched; and once the seizure was joined, the two of them would have killed each other and both been lost. Then you, the third party, would have had all the profit, stealing into my body at your ease and winning the day. Am I not right in this, my great adept Yu Zitong?" And Han Li, in one breath, calmly set out his judgment.
Yu Zitong, hearing this, was silent for a long while; at last he sighed, and, somewhat disheartened, made no denial.
"Before, when I sang your praises, I spoke at random; but now I praise you in all earnest. You are truly clever — you have gone beyond the teacher and bettered the art, outwitting the fox that was Mo Juren.
"You have guessed aright; all this, indeed, was of my devising. But I had never thought your aptitude for cultivation so fine that, in this brief time, you should climb to the sixth layer of the Art of Eternal Youth, but one layer below my own — that you should swallow up Doctor Mo's spirit without the least trouble, and that even my own spirit, mangled as it was, should prove no match for yours, and go the worse again of the encounter."
But his tone turned, and suddenly waxed proud:
"Mo Juren was but a mortal of the common herd, and would needs sit as an equal to us cultivators, and call us brothers! Was he worthy of it?"
"Worse still, he brought my true body to ruin by base means, and would still set his foot upon the immortal path — a fantasy dreamt by daylight, were it not?" And Yu Zitong gnashed his teeth again, plainly nursing a hatred of old against Doctor Mo, which now he flung forth without the least restraint.
"But you are different. You were born with a spiritual root, and are gifted beyond the common run; in the mortal world, it is a great pity. If you would but find me a fitting body and help me accomplish my seizure, I would be your guide, and bring you before the elders of my clan that they might take you as their disciple. What say you to that?"