When a Major disappears after an skirmishes with enemy forces, a search and rescue is undertaken as a Rookie slowly uncovers the existence of a greater, more primal threat that evolves into a series of horrific encounters beyond their capabilities.

>>>as an ancient world adversary is uncovered

That's the story hook and the inciting incident that kicks off the plot so it needs to be described more clearly.? ?What is so unique about the adversary that makes it of dramatic interest, that makes it worthy of being a "horrible" being in a horror flick?

Let's consider the formula's 3 elements.

I think the confusion starts from choosing the Major's disappearance as the story's Event (call to adventure). Yes, the story start like this, but the actual Big Event is the creature's discovery and the fact that it is killing them off one by one.

Thus, I would start the logline with "When they uncover [an ancient killing giant] during a rescue mission..." or something similar.

Then, the character: "... a [something something officer]..."

Then the action: "... must [survive from its teeth]."

OK, my choice of words sucks bigtime, but I hope it helps you see the 3 elements you need to nail.

The logline sets up a situation, but now it needs to follow through with? the missing ingredient for a plot:? what happens after the discovery?? What becomes the objective goal?? What is the action that arises from the encounter?