As Jerik stumbled through the Eastern Forest, he had to stop several times to wretch up what was left in his stomach.
Some nit-picky stuff, not much because I'm time-limited right now:
Personally I would prefer a sentence structure that was more direct, but that's more a matter of style and personal preference. So the above, as written, might become:
Jerik stumbled through the Eastern Forest. He stopped several times to wretch up what was left in his stomach.
But there are other issues here that give an impression of sloppiness in the writing. For example:
He stopped several times to wretch up what was left in his stomach.'Wretch' refers to a person, not the act of vomiting. For that you almost certainly mean 'retch', so better is:
He stopped several times to retch up what was left in his stomach.
Still not great, because vomiting by its nature involves bringing up the contents of your stomach, so it only needs to be:
He stopped several times to retch.
But retch typically suggests the act of vomiting whereby your body heaves but you don't bring anything up (perhaps because there's nothing left to bring up). So if you mean the contents of the stomach being brought up, you probably don't want to use retch at all. Maybe what you're really after is simply:
He stopped several times to vomit.
The venom that coursed within his veins sickened his body and clouded his mind.
I'm not sure why you use within when the simple in would do:
The venom that coursed in his veins sickened his body and clouded his mind.
It's implied that the venom is in his veins if it's having an effect, so you could reduce it to:
[The] venom sickened his body and clouded his mind.
It still conveys what you're after.
He had delusional visions playing within his mind, causing him to see thousands of eyes staring at him from the countless trees around him.
Again, more style and personal preference, but perhaps:
Delusional visions played in his mind, causing him to see thousands of eyes staring at him from the trees.
He wiped the edge of his mouth a looked up at the full moon. It was so brilliant he had to squint his eyes, causing a shooting pain to gnaw at his temples. He shivered against the cold night and hugged his tired arms tightly around his torso.
Not sure why you need him wiping his mouth at this point. Doesn't seem to add anything useful and it draws attention away from the bright moon and the cold, which is what you probably ought to focus on.
Hope that helps.
Omni