Short story (70s/80s?) read in a magazine like Analog, maybe 20 years ago.
The aliens live on a dying planet, feeding around a volcanic pit. They control the protagonist to construct a portal, granting him mind-control abilities. He becomes aware of the plot and sends a bomb back through the portal to destroy the aliens. He teleports to a quiet beach at the conclusion of the story.
Sounds like End as a Hero by Keith Laumer.
The protagonist, Granthan, wakes up in a lifeboat as the first survivor of an attack by the enemy Gool.
"... you Psychodynamics people have been telling me the Gool may have some kind of long-range telehypnotic ability that might make it possible for them to subvert a loyal man without his knowledge. You've told me yourself that you blacked out during the attack—and came to on the lifeboat, with no recollection of how you got there.
"This is war, Granthan. War against a vicious enemy who strike without warning and without mercy. You were sent out to investigate the possibility of—what's that term you use?—hypercortical invasion. You know better than most the risk I'd be running if you were allowed to pass the patrol line.
"I'm sorry, Granthan. I can't let you land on Earth. I can't accept the risk."
He manages to force mental contact back over the link the Gool have implanted in him:
I saw a world of yellow seas lapping at endless shores of mud. There was a fuming pit, where liquid sulphur bubbled up from some inner source, filling an immense natural basin. The Gool clustered at its rim, feeding, each monstrous shape heaving against its neighbors for a more favorable position.
I probed farther, saw the great cables of living nervous tissue that linked each eating organ with the brain-mass far underground. I traced the passages through which tendrils ran out to immense caverns where smaller creatures labored over strange devices. These, my host's memory told me, were the young of the Gool. Here they built the fleets that would transport the spawn to the new worlds the Prime Overlord had discovered, worlds where food was free for the taking. Not sulphur alone, but potassium, calcium, iron and all the metals—riches beyond belief in endless profusion. No longer would the Gool tribe cluster—those who remained of a once-great race—at a single feeding trough. They would spread out across a galaxy—and beyond.
And yes, he teleports himself to a beach at the end:
I opened my eyes, raised myself on one elbow—and saw the sea. The sun was hot on my body, but not too hot, and the sand was white as sugar. Far away, a seagull tilted, circling.
A wave rolled in, washed my foot in cool water.
I lay on my back, and looked up at white clouds in a blue sky, and smiled—and then laughed aloud.
Distantly the seagull's cry echoed my laughter.
