Martin didn’t remember what had awakened him. On an average night, he would have assumed it had been the hooting of an owl or the crackling of a fire. Without even giving it a second thought, he would go back to sleep. But this was different. His awakening on this particular night was sudden and abrupt as opposed to his usual calm and heavy-eyed reaction to a midnight stirring. The muscles in his arms were tense and his ears throbbed in their effort to pinpoint the source of what had awakened him with such a startle. Commonly, Martin would simply not care about what had awakened him. He would rarely even open his eyes in the middle of the night, and would simply rub them before going back to sleep. Now his eyes were wide open as he strained to see in the dark.
After what seemed hours of sitting in the night, his throbbing ears took notice of a sound. It was a very quiet sound, and almost impossible to hear. It was the sound of a bare foot quietly setting down on the dirt ground. He listened for a second step, just to make sure it wasn’t his imagination. It came; only this time it was louder as the footsteps neared his tent. The hair on the back of his neck stood straight as a chill went up his spine. Someone was coming.
Another comment - I've seen a zillion openers now of people waking up, and I've only been on this forum for a couple of months. It would be far more interesting if, when the character hears the sound, instead of sleeping he has been engrossed in something that conveys some personal quirk or characteristic - maybe hand carving chess pieces, or painting or something like that.
Just my 2 cents.
Now, THAT would hook me!! You should toss something in that mentions an alternate dimension. It would take your ordinary opener and turn it into something more extraordinary.
It bothers me that he was supposedly wakened by a sound. He then listens for "what seemed hours" before detecting a sound so quiet he could barely hear it. Granted that his time sense is distorted, it still seems like he must have been listening for minutes; and if he can barely hear the footsteps now, when they've gotten closer, how could he have heard them minutes ago loudly enough to wake him up. Also, it feels like a substantial period of time passes between the first footstep and the second, but: "It came; [comma would have been better here] only this time it was louder as the footsteps neared his tent" makes it seem that a whole pitter-patter of footprints were almost simultaneous with the second one. Something like: "This time it was louder, closer" would work far better. The last sentence, "Someone was coming," doesn't add anything--I think we've figured that out by now.
But most the writing itself is clear. I agree with the above comments on letting us know about the milieu in a hurry.