Line 29

There wasn't a breath in that land of death, and I hurried, horror-driven,

• All of a sudden, things have really changed. Up until now, we were on what felt like kind of a fun adventure. Now, it feels more like a horror movie.
• The winter world has turned into a "land of death." It’s completely still and lifeless ("without a breath").
• Our speaker really gets moving now, feeling "driven" by a feeling of horror.

Line 30

With a corpse half hid that I couldn't get rid, because of a promise given;

• What makes our speaker feel so horrified? Well, he’s got the frozen corpse of his friend peeking out ("half-hid") from his sleigh.
• He can’t bury Sam or leave his body behind, because of that promise he made. He's got to find a way to burn Sam's corpse.

Line 31-32

It was lashed to the sleigh, and it seemed to say: "You may tax your brawn and brains,
But you promised true, and it's up to you to cremate those last remains."

• Now, things start to get weird. The speaker has Sam’s body "lashed" (tied down) to his sleigh, and he starts to almost imagine that it’s talking to him.
• The corpse mocks the speaker, reminding him that he can try to work his way out of it with his muscles ("brawn") or his brain, but a promise is a promise. He has to burn the body.
• Remember that they are in the Arctic. It’s not easy to even start a fire, much less burn a whole body, so we’d say this is a pretty serious inconvenience.