Slimer

One by one it took them... and death was only the beginning!

—Cover Blurb

Slimer is a science-fiction/horror novel by "Harry Adam Knight," a.k.a. Australian author John Brosnan, writing under a pseudonym. Brosnan also wrote the novels Carnosaur and The Fungus under this name.

Paul Latham, his girlfriend Linda Walker and their four friends Alex, Rochelle, Mark and Chris had been on a drug run from Colombia when their yacht sank. They had to abandon ship so quickly they lost all of their drugs (except for Alex's personal stash of heroin). By a seeming stroke of luck, they stumble across a seemingly abandoned oil rig. Going aboard, they discover the rig is actually a cover for a top secret lab run by a company called Brinkstone.

Some exploring reveals discarded, unfired guns and scattered, empty clothes. That night, a mysterious, unseen creature attacks them. Although Paul empties an M16 into it, this doesn't seem to harm it. They come across a scientist, Dr. Gordon Shelley, who says the creature is nicknamed "Charlie" and won't harm them again. Although he promises he'll explain more later, he is missing the next day. Later a security officer confronts the group, offering to help, but before their startled eyes turns into "Charlie," a monstrous creature which wounds Rochelle.

Paul and the others eventually discover that "Charlie" is actually a great white shark given shapeshifting abilities by an experimental compound called "Phoenix." It absorbs the minds and bodies of its victims, and can take their shape, and they're next. To make matters worse, the increasingly hostile Alex begins antagonizing everyone at every turn, questioning Paul's leadership and threatening to be as dangerous to the group as the monster itself.

Slimer was adapted in a 1995 film called Proteus by Brosnan himself, who in addition to changing both the title and the name of the compound which creates the monster, also switched around characters' names and roles, making Alex of all people the likable DEA agent hero! While mostly a generic Alien ripoff, Proteus still has some good moments and is a far more faithful Brosnan adaptation than the earlier Roger Corman Carnosaur.

Not to be confused with the ghost of the same name from the Ghostbusters franchise.

Tropes used in Slimer include:

Accidental Murder: Paul accidentally shoots one of the rescue chopper pilots at the end, mistaking him for one of Charlie's forms.
And I Must Scream: The fate of Charlie's victims.
Apocalyptic Log: Dr. Shelley's video tapes about the Phoenix experiment.
Apocalypse How: The downer ending wherein Charlie, in the form of the second pilot, falls into the ocean, implies that should it survive its heroin withdrawal symptoms (see Weaksauce Weakness below) it could go on to reproduce itself and absorb everything and everyone on Earth and supplant the human race entirely.
Asshole Victim: Alex. And Mark after his Heroic BSOD causes him to turn into a real Jerkass.
Eagle Land: Alex Rinaldo is the lone American in the group (a Mexican-American to be precise, not that this seems to make much difference) and at one point touts the US as "the greatest country in the world." The American helicopter pilot at the end also brags that "we have the death penalty where I come from!"
Empty Piles of Clothing: All that is left after Charlie absorbs its victims. Interestingly, Charlie has the ability to morph clothing, as when the absorbed people's minds surface and alter Charlie's form to "themselves," they don't appear naked, but wearing clothing that is attached to their bodies and can't be removed.
Everything's Even Worse with Sharks: A great white to be precise. A mutated, super-intelligent shapeshifting one.
Extra Eyes: When in the form of the second pilot Charlie grows an extra eyeball in the back of his head to watch Paul and Linda in the back seat. Creepy.
Face Full of Alien Wingwong: After Charlie evolves from absorbing individual people into itself and moves on to "converting" humans into more of its new mutant shark species, it uses a penis-like barbed claw dripping a fluid to "impregnate" the helicopter pilot and later tries to do this to Paul aboard the copter when impersonating the pilot. It even refers to this as "giving its seed" to people.
The Ghost: Lloyd Brinkstone, the owner of the Brinkstone company, who is mentioned several times in Shelley's video tapes but never actually appears. He does however appear in Proteus.
Gone Horribly Right: The Phoenix experiment, according to Shelley. They worked hard to design something that could survive almost anything and boy howdy, they sure succeeded!
"I Know You're in There Somewhere" Fight: Inverted to an extent. The people Charlie consumes have their minds and memories absorbed into him, but they can still force themselves to the surface and use Charlie's shapeshifting ability to make it look like them so they can try to warn others. However, this trope is done by the absorbed people to themselves and no one outside Charlie ever attempts this.
Kill It with Fire: Tried against Charlie. It hurts it, but doesn't ultimately do any lasting damage. In Proteus the trope is played straight and this succeeds in killing it at the end.
Heroic BSOD: Mark suffers one when his girlfriend Chris is taken by the creature.
Twenty Minutes With Jerks: Except for levelheaded Paul, everyone is either a thoroughly unpleasant person or never stops whining.
Weaksauce Weakness: The seemingly invincible Charlie's weakness turns out to be drugs, which is discovered when it absorbs heroin junkie Mark, who had just shot up. The mutant is essentially defeated by being turned into a heroin addict!