Downward to the Earth - Robert Silverberg

 Interesting concepts abound here.   A decade after the planet was restored to its sentient people, Edmund Gundersen feels a compulsion to return to Belzagor to put things right on his own account.   A former worker for the colonial power, Gundersen was peripherally involved in some of the misdeeds that went on.   He encountered the notorious Kurtz (yes, Kurtz) who shared snake venom with one of the two sentient people, the elephantine nildoror.   Gundersen once saw Kurtz, venomed up to the gills himself, dance with the nildoror.   Gundersen's time on Belzagor wasn't all bad, though.   He met and fell in love with the beautiful Seema.

But now Seema is partnered up with Kurtz.   Kurtz, she says, is off on an expedition, but her sulidor (the sulidoror are the other sentient people on Belzagor, giant ape-like people) tells Gundersen that Kurtz is hidden away on the compound, ill.    Gundersen sees him - and is horrified.

It is rare for a planet to have two sentient races, particularly two races so strikingly different.   Both have speech, both are said to have souls.   The land is divided between them.   The nildoror have the fertile plains, the sulidoror occupy the misty uplands.   The nildoror are vegetarian grazers, the sulidoror omnivore hunter-gatherers.   There is no emnity: the two races come and go across one another's territory   Both have seemingly come to terms with their colonial past in which the sulidoror were servants, the nildoror transport.   They continue to provide these services for human tourists.   Now they do it by consent.

Gundersen has always got on reasonably well with the natives of Belzagor.   He can speak both languages, though he is not so fluent with the gestures that provide nuance.   He politely seeks permission from a senior nildor to go to the hill country.   Permission is granted so long as he brings back Cedric Cullen, who has apparently commited a serious transgression and exiled himself among the sulidoror.   Gundersen wants to find out what happened to Kurtz in the hill country.   Most of all he wants to find out about the rebirth ceremony the nildoror undergo there.   Every twenty years ot so a nildor is summoned to the rebirthing.   This was Gundersen's transgression: he needed nildoror to help repair a breached dam and prevented them going on their rebirthing trek.    So off he goes - into the heart of darkness.

Yes, Downward to the Earth is a sci fi take on the Conrad classic.   The question in both is what has Kurtz found that has turned him into a monster?   'The horror ...  the horror...."    Silverberg's version pays off big time, with a twist that I absolutely didn't see coming.   This is the first time I have read any Silverberg.   I was very impressed.