Fresleven was a boat captain who worked for the Company, and his death created a vacancy that Marlow is hired to fill.
Fresleven, a Dane, is described as a peaceful and gentle person, yet his death came about by his own actions; while in the African interior, he quarreled with a tribal chieftain over a matter of two chickens. When Fresleven began beating the aged chief with a stick, one of the tribesmen, presumably the chief's son, killed Fresleven. The villagers fled, abandoning Fresleven's body and the village, and Marlow finds his body, untouched, but reduced to bones, lying where he died, nearly hidden by the grass that had grown there.
There are two ironic aspects to Fresleven's fate. One is that he is described as the "gentlest, quietest creature that ever walked on two legs" - and yet he mercilessly beat an old man with a stick. The second ironic aspect is more of a metaphor. The villager who killed Fresleven is described as making a "tentative jab" with his spear - perhaps suspecting that he will be unable to kill the white man because of some terrible power the white man possesses. Indeed, the villagers immediately flee when they see Fresleven is dead, fearing that some calamity is sure to ensue. Yet, Fresleven lies where he fell, his bones overgrown; while in life he may indeed have been a powerful European, embarking upon a "noble cause" as Marlow sarcastically describes it, in death he is little more than jungle fertilizer.