Seoul — During his long, unchallenged rule stretching back to the 1940s, Kim Il Sung has fashioned an unmistakably unique brand of communism in North Korea. On top of a highly developed statewide personality cult, he now appears intent on rewriting Marxist-Leninist theory by creating its first dynasty.
Having celebrated his 71st birthday last April, he has spent the last few years grooming his trusted son, Jong Il, to take over power.
The process has not always gone smoothly, and close obervers of North Korea consider there is a strong element of opposition to the son. But still, on Jong Il's 40th birthday in February last year, he was awarded the title of ''hero of the nation.''
And to the 18 million North Koreans, while Kim Il Sung is known as ''the great leader,'' the son has now become ''the dear leader,'' according to recent foreign visitors to Pyongyang, the North Korean capital.
Visitors to Panmunjom, the truce village in the demiliarized zone between North and South Korea, have noticed an addition in recent months to the uniform of northern soldiers - a badge showing the head of Jon Il similar to the long standard one of the father.
Experts here say the East German-educated Jong Il has been steadily taking over the reins of power from his aging father, whose health is suspect.
A significiant step forward, they say, was the son's official visit to China in June, which was only belatedly revealed by the Chinese - who are not keen on personality cults these days, but accepted the younger Kim apparently to strengthen their influence in Pyongyang and offset the longstanding dominance of the Soviet Union.
The change of leadership from father to son is not something South Korea feels particularly happy about. At least, say southerners, Kim Il Sung is a known quantity they have learned to live with.
Il Sung returned home in 1945 as a resistance hero against Japanese colonial rule of the Korean peninsula. Under Russian tutelage, he established the Democratic People's Republic of Korea in 1948, at the same time as the pro-American Republic of Korea emerged in the south. He has ruled ever since, either under the title of prime minister or head of state until a new constitution in 1972 established the post of president.
Although Kim Il Sung sent his troops south in 1950 to start a three-year war and has never for a moment given up his resolve to reunify the country on his own terms, still there are some analysts in Seoul who believe he has mellowed and has lost some of his belligerency.
In the past few years, intelligence experts note, trends on both sides of the 38th parallel have tended to induce North Korea to try and rely primarily upon political and diplomatic measures to further the objective of reunificiation.
But that same period has seen South Korea recover political stability and become economically much stronger - a prosperity that also helps strengthen its military. Even so, experts reckon it will be at least six or seven years before the South can achieve any sort of parity with the heavily armed North. South Korea also has scored some major international successes in winning the right to hold the 1988 Olympic Games, staging major international conferences, and opening up greater contacts with the communist and nonaligned worlds.
All this, the analysts say, has tended to turn the North back toward a more militant posture, heightened by recent political developments like the emergence of Kim Jong Il and the increased power of the military in government policies.
South Korean sources repeatedly refer to a dangerous streak of ''adventurism'' in Jong Il, and feel that in order to cement his hold on power after his father's disappearance he will have to maintain a high degree of militancy that could add even more tension to the Korean peninsula.
They worry, for example, that Jong Il lacks the international revolutionary contacts that have been something of a restraining influence on his father in recent years. Both China and the Soviet Union appear to have held back Kim Il Sung on several occasions when he reportedly sought their support for a military move on the South, officials in Seoul claim.
But said one official: ''Jong Il is very much a creature of the sycophantic domestic climate. There is no real outside influence to balance any irrational behavior.''
South Korean Defense Minister Yoon Sung Min, bluntly warned his military commanders the other day that the growing power of Jong Il would increase the risk of ''provocative acts'' like the Oct. 9 bombing in Rangoon which decimated Korean government ranks and which Seoul has firmly blamed on North Korea.
Analysts say there remains a very real question whether Jong Il will be able to establish his credentials to control the government and party after the departure of his ''illustrious'' father. Some experts see a power struggle breaking out in which the support of the military will be crucial.
A recent private American study noted that ''Jong Il's succession likely will be accompanied by heightening of political and ideological objectives and increased motivation to maintain a high level of military effort. . . . The process of succession will increase the range of uncertainty regarding North Korean behavior.''
In this regard, analysts recall a period in 1976 when Jong Il temporarily took over power when his father was ill. Northern rhetoric reached vituperative heights not seen for a long time. It was also at this time that two American soldiers were axed to death at the Panmunjom truce village by northern troops, provoking an immediate strong American military buildup in the south.
The younger Kim disappeared from public view for almost two years, leading to rumors that he had been seriously hurt in a car accident or that he was having health problems. The prevailing theory now is that he was put out to pasture for a while because he had proved he was not ready to run the country safely.
But he has steadily taken on a larger role in national life ever since reemerging in 1978, provoking in many South Koreans the depressing feeling they can never escape from the bogeyman named ''Kim'' undermining their search for peace, security, and a unified Korean nation free from ideological confrontation.