Marlow faces several "tests" on his trip up the Congo River in Heart of Darkness by Joseph Conrad.
When Marlow first arrives at the Lower Station, he has to learn to cope with the harsh reality he is presented with. Here Africans (many of whom are starving and dying) have been enslaved by the white Company men. The most important thing to them is ivory.
The word 'ivory' rang in the air, was whispered, was sighed. You would think they were praying to it.
A wreckage of rusting ships float in the harbor. Wanton destruction takes places around him: it is "vision of hell." The whites practice absurd behaviors, and act as if "bewitched." The entire place is an enigma to Marlow: he can make little sense of what he sees, perhaps in comparing it to his life in England.
Another "test" is trying to repair the ship he is supposed to command. When he arrives at the Central Station, it has sunk into the river. He is told it will take an extended period of time to repair it, but notices that over the weeks as he tries to do so, his attempts seem to be thwarted: materials he needs to fix the boat are not arriving, and he suspects that the manager is doing everything in his power to keep Marlow from completing his task.
The last test revolves around securing "cargo." Marlow eventually gets underway, heading for the Inner Station where he is supposed to pick up Kurtz, perhaps the most successful man in the Company at collecting and shipping out enormous amounts of ivory, and bring him back.
At one point Marlow's ship is fired upon with arrows by natives on the shore. They find a hut that has a sign warning them to use care in moving forward. Upon arriving, Marlow learns that Kurtz is worshipped by the natives, but he is also someone who has lost sight of civilized behavior. As Marlow and his men reach Kurtz's house, they see poles surrounding it.
Now I had suddenly a nearer view, and its first result was to make me throw my head back as if before a blow. Then I went carefully from post to post with my glass, and I saw my mistake...They would have been even more impressive, those heads on the stakes, if their faces had not been turned to the house. Only one, the first...was facing my way.
Kurtz, a charmer, has succumbed to the "powers of darkness," to become a part of this alien culture. The natives are "his people," and he is their revered leader. In this comes the last part of Marlow's test: to convince Kurtz, when they finally meet, to leave with him.
Kurtz is slowly convinced, changes his mind and returns to the jungle, and is finally convinced again. Marlow loads Kurtz onto the ship, and laments rise from the natives left behind. Kurtz is very ill and does not survive the journey. They bury him along the shore.
This is a great question because it identifies how Conrad in this novella is playing with a central archetype that is present in so many works of literature - the journey and the protagonist who faces a series of challenges as he goes on his journey. Whether we look back to works such as The Odyssey or to contemporary works of fiction such as The Road by Cormac McCarthy, the journey is an archetype that has found many different forms over the years.
What is interesting though about this novel is that in some ways Marlow is more an observer of the follies and horrors of colonialism rather than a character who faces challenges and needs to overcome them. Many argue that the journey Marlow makes into the heart of darkness is richly symbolic and could be read in psychoanalytical terms as Marlow ventures more and more into his own inner psyche. The terrors he narrates to us and especially his encounter with Kurtz make him realise more about who he really is as a human being and what he himself is capable of. This would be the central challenge that Marlow faces at the end of the novel. Note how he comments upon it:
However, as you see, I did not go to join Kurtz there and then. I did not. I remained to dream the nightmare out to the end, and to show my loyalty to Kurtz once more. Destiny. My destiny! Droll thing life is - that mysterious arrangement of merciless logic for a futile purpose. The most you can hope from it is some knowledge of yourself - that comes too late - a crop of unextinguishable regrets.
Having met Kurtz, Marlow is forced to come to terms with his own "crop of unextinguishable regrets" and this is a battle that nearly costs him his life, but he is able to "bring his foot back from the brink," unlike Kurtz, who stepped over.
Thus the central challenge that Marlow faced is recognising his ability to commit the same kind of crimes as Kurtz and to fall into the same kind of lawless anarchy.