I think it is a classic 50's story because I think at the time I was reading a series of compilation books... Something like "50's greatest Science Fiction Stories" or maybe Award Winning short stories from different decades. As I remember, the protagonist walks around some outpost where there is a race of nonspeaking rulers that dominate the place but no-one can make heads or tails of. It isn't until the climax when it is revealed that the rulers' brains had been pretty much replaced by fungi.
In this story, Brion Brandd is the champion of a planet-wide "Games" involving physical and mental competition from many different events from sword-fighting to chess, on a planet that has extreme swings in climate. As the champion is physically fit and extremely adaptable. The games are known as the "Twenties":
The Twenties were more than just a contest—they became a way of life that satisfied all the physical, competitive and intellectual needs of this unusual planet. They were a decathlon—rather a double decathlon—raised to its highest power, where contests in chess and poetry composition held equal place with those in ski-jumping and archery.
On winning he is visited by a former champion of the same "games' and told of a problem on another planetary system where one planet (Njord) is threatening total destruction of the other (Dis). Dis is inhospitable - hot, dry, all water is underground, and the "native" people are extremely inhospitable and violent - as a result of their evolution to fit the climate from colonization. Despite this, the Disans have managed to get a bomb launcher capable of sending bombs to Njord and are demanding instant and unconditional surrender and are not willing to accept anything else, to which the Njordians are sending their own (space) fleet with the intention of wiping out all life on Dis.
Brion lands on Dis a with a Xenobiologist (Lea), but their ship is destroyed by Disans before they can meet their working group from the recently established city (new colony), but in walking across the desert he manages to befriend one of the Disans after nearly dying from dehydration.
In the city he finds that there are a group of Disans known as Magte (plural: Magter), who are political and religious leaders of the Disans:
Magte is a title that means roughly noble or lord. Lig-magte is the local overlord.
The Magter are known for living in isolation, being particularly uncommunicative, emotionless and extremely violent and merciless - reacting to any negative with an attack intended to kill and will kill unless killed themselves. It turns out, from his Disan friend that the Disans would like to not bomb Njord, but the Magter insist and hold all the power. Brion visits the Magter in an attempt to discover their motives, but is attacked and manages to escape.
With a terrible sinking sensation Brion then realized what would happen and what he had to do. Lig-magte was as heedless of his own life as he was of the life of his planet. He would press the attack no matter what damage was done to him. Brion had an insane vision of him breaking the man's other arm, fracturing both his legs, and the limbless broken creature still coming forward. Crawling, rolling, teeth bared, since they were the only remaining weapon.
Brion then does some logical thought and comes to the conclusion they aren't human (any more):
I've been visiting," Brion said, forestalling the question on her lips. "The magter are the ones who are responsible for causing the trouble, and I had to see them up close before I could make any decisions. It wasn't a very pleasant thing, but I found out what I wanted to know. They are different in every way from the normal Disans. I've compared them. I've talked to Ulv—the native who saved us in the desert—and I can understand him. He is not like us in many ways—he certainly couldn't be, living in this oven—but he is still undeniably human. He gave us drinking water when we needed it, then brought help. The magter, the upper-class lords of Dis, are the direct opposite. As cold-blooded and ruthless a bunch of murderers as you can possibly imagine. They tried to kill me when they met me, without reason. Their clothes, habits, dwellings, manners—everything about them differs from that of the normal Disan. More important, the magter are as coldly efficient and inhuman as a reptile. They have no emotions, no love, no hate, no anger, no fear—nothing. Each of them is a chilling bundle of thought processes and reactions, with all the emotions removed."
Lea tried to shake the knots from her drug-hazed mind. "I'm dull today," she said. "You'll have to excuse me. If these rulers had no emotional responses, that might explain their present suicidal position. But an explanation like this raises more new problems than it supplies answers to the old ones. How did they get this way! It doesn't seem humanly possible to be without emotions of some kind."
"Just my point. Not humanly possible. I think these ruling class Disans aren't human at all, like the other Disans. I think they are alien creatures—robots or androids—anything except men. I think they are living in disguise among the normal human dwellers."
To prove his theory Brion then attacks the Magte compound and captures a corpse - he then takes this back to his Disan friends for a dissection, which finds that:
"Do you see those spherical green shapes grouped together?" Lea asked. Before Brion could answer she gasped, "I remember now!" Her fatigue was forgotten in her excitement. "Icerya purchasi, that was the name, something like that. It's a coccid, a little scale insect. It had those same shapes collected together within its individual cells. Her eyes opened wide as she caught the significance of her own words. A symbiote—and Dis was the world where symbiosis and parasitism had become more advanced and complex than on any other planet. Lea's thoughts spun around this fact and chewed at the fringes of the logic. Brion could sense her concentration and absorption.... ...The magter's brain was only two-thirds of what would have been its normal size. Instead of filling the skull completely, it shared the space with a green, amorphous shape. This was ridged somewhat like a brain, but the green shape had still darker nodules and extensions. Lea took her scalpel and gently prodded the dark moist mass.
this leads to Brion and Lea showing the parasite to the Disans, who then declare the Magter as umedverk, meaning "one who is out to destroy you", and as a result declares an automatic death sentence to all Magter, which eventually saves the planet.