Power Parasite

Contrary to popular belief, certain villains don't have the same sort of powers, stamina or durability that the heroes do. Maybe they were hit with The Worf Effect in the backstory, maybe they lost their abilities sometime earlier, maybe they recognize that they genuinely cannot fight without an upgrade, or maybe they're literally power-hungry.

Whatever the case, these villains are Dangerously Genre Savvy enough to know that, if they can't beat 'em, be them. In direct defiance of the Superpower Lottery, and through manipulating the Sorting Algorithm of Evil, these villains completely absorb, implant or outright steal someone elses' abilities as their own, giving them a better chance at fighting their enemies. Said new abilities may or may not have an affect on the overall health or mental condition to the villain in question. In terms of Playing with a Trope, when your whole ability is to steal other abilities think of it as a Power Sponge, especially if it ends up like a black hole, but such examples are rare.

Contrast Power Copying, where you learn an ability by seeing it done, and compare Cannibalism Superpower, where eating someone allows access to their abilities. If the Power Parasite can do this to more than one person at a time, it's All Your Powers Combined, which might give way to a reverse-Final Exam Boss scenario. Whether or not they actually succeed in defeating anyone (or maintaining their sanity) is up to the author.

This is the Opposite Tropes of Super Empowering. More often than not, such an ability is a secret twist in the story, so these are Unmarked Spoilers.

Anime and Manga

 * Bleach takes an unusual twist by having the Power Parasite, Kugo Ginjo personally train The Hero to achieve a power (Fullbring) suitable for absorption to use as an upgrade. Upon accomplishing this, Ginjo gained a skeletal set of armor, along with the ability to grant upgrades to his fellow Fullbringers by slashing them with his sword.
 * Aaroniero Arruruerie can absorb anyone into him, like Metastacia. Since Metastacia possessed Kaien Shiba at the time, Aaroniero also absorbed Kaien. He used Kaiens' face and his Zanpakuto to fight against Rukia.
 * In Claymore, Roxanne of Love and Hate became the number one of her generation by copying the powers of every higher-number warrior she's been paired with, then letting them die in an accidental fashion.
 * In Hunter X Hunter, Chrollo Lucilfer is a Power Sponge - his only ability is to steal the complete set of powers from another person; the victim is effectively Brought Down to Normal for the rest of his or her life (and cannot even sense other people with powers, a trait even some non-supers can do in this series). Chrollo has a tougher time than most to obtain these powers, as he needs to meet the person whose powers he's going to steal, Chrollo needs to hear the name of the power from said person and see it performed, needs to summon his book which stores the powers to hold it in, and needs to perform the above within 24 hours or that person becomes immune to his powers. In addition, he needs to hold the book out in order to use these powers. Chrollo is Dangerously Genre Savvy enough to have collected a number of powers from fighters stronger than him.
 * In Naruto, Tobi, so very much. In addition to stealing Izuna's Mangekyo Sharingan, he's acquired Izanagi through samples of Hashirama's DNA and stolen Nagato's Rinnegan. Through the Rinnegan, he replaced the Six Paths of Pain with the six deceased Junchuriki, mastered Edo Tensei and controls the Demonic Statue of the Outer Pain.
 * Kabuto Yakushi may be an even worse offender. The "Edo Tensei" summoning involves acquiring the DNA of a corpse that you intend to revive, so virtually anyone can use it, as did Tobirama and Orochimaru. Kabuto perfected it, by reviving every single deceased high-profile shinobi in the series, from the top dogs like Jinchuriki, Kages and Akatsuki members, to the Seven Swordsman of the Mist, well-known Jonin like Dan (Tsunade's dead boyfriend), criminals like Ginkaku and Kinkaku, side villains like Hanzo and Kimimaro, and finally, . In essence, Kabuto's "stolen" their corpses (and their corresponding abilities), and forced them to act as Tobi's reinforcements, without even having to steal anything personally.
 * A lesser-level example is Danzo Shimura, who embedded several Sharingans on his arm and used pieces of Hashirama's DNA to get Izanagi.
 * One Piece: Marshall D. Teach is one of the stand-out cases, since he absorbed Whitebeard's Devil Fruit by using his own darkness/gravity powers. Especially notable since he's the first to ever achieve such a feat in the entire series, and it's treated very seriously. Worse still, two years later he's still at it, which means he (and probably his crew) more than likely has even more powers now.
 * To Aru Majutsu no Index: Fiamma of the Right severed Touma's hand and absorbed it to gain the power of Imagine Breaker, purifying his Holy Right and unlocking its full potential. Unfortunately for him, Touma's willpower was enough to get his hand back.

Comics

 * X-Men member Rogue has this as her superpower; depending on how long she touches a victim, it lasts from minutes to forever. In her first appearance, she got her Flying Brick ability by permanently absorbing it from Ms. Marvel. Since Rogue's power also absorbs memories and personality traits, it took Ms. M years to recover.
 * The so-called Mutant Messiah Hope has a similar power, being able to "borrow" (consent optional) the powers of other mutants.
 * Doctor Doom does this a lot. Most famously, he briefly stole the Silver Surfer's power back in the 1960s, but on later occasions he's also stolen energy from the Beyonder (in Secret Wars), and from Aron the Rogue Watcher and the Marquis of Doom (in Fantastic Four).
 * The Spider-Man villain The Vulture can do this in addition to draining his victims' youth Depending on the Writer.
 * Depending on the Writer, minor Iron Man villain the Controller can sometimes steal powers with his Mind Control technology; even then, it only works on psychic powers.
 * Superman foe the Parasite has this as his shtick. It comes with a time limit, so he has to continuously drain a person's power to have it. In keeping with the "doesn't necessarily make them winners" aspect of this trope, he's not that dangerous (being defeatable by simply staying away from him does that) unless paired with other villains to back him up, and he'll betray them if they suggest killing Superman.
 * The latest threat that Buffy the Vampire Slayer faces in the Season 9 comics is a Humanoid Abomination called "The Siphon" who can absorb the power of any supernatural being -- including the Slayers.

Film

 * The nerdlucks from Space Jam transform into the Monstars via this tactic; specifically, by stealing the talent from professional NBA players.

Literature

 * In the Mistborn series this is what Hemalurgy does. By killing a person with a metal spike and implanting that spike in your own body, you can steal one Allomantic, Feruchemic, or human power from them. Some Hemalurgists, like the Steel Inquisitors, might have up to twenty spikes.
 * In Harry Potter, Voldemort steals Dumbledore's wand by the last book. It's a lot more mundane than the other examples, but its effects were still deadly.
 * Mostly heroic example. Harry Keogh in the last of the original Necroscope books, "Deadspawn". Spends the entire book taking other people's powers (to be fair most of them weren't needing them since they were dead at the time) for the final confrontation with the Big Bad in the alternate universe. It Makes Sense in Context.
 * In an example of this type of parasitism similar to the example below, the Yeerks from Animorphs will use other creatures as hosts for their specific abilities.

Live Action TV

 * In Heroes, this is what Peter Petrelli's power started as, and he qualifies as a Power Sponge.
 * Gabriel Gray/Sylar is the archetypical Heroes example of this process, since most of the people he stole from wound up dead, as a result.
 * The Goa'uld of Stargate SG-1 fame could be considered a strange example of this, as while they can't steal powers directly, they can possess people who do have those powers. One villain, Ba'al, tried to do this with season ten villain Adria, who had god-like powers. Unfortunately, those same god-like powers prevented her from being taken as a host. This is also the reason why Goa'uld sometimes take hosts that aren't human- when they take Unas as hosts, they do so because the Unas are far tougher than humans, but their bodies are more difficult to control and repair.

Tabletop RPG

 * In Champions, characters can obtain this ability by purchasing Power Transfer.
 * Mayfair Games' DC Heroes used Power Drain for this ability.
 * In Vampire: The Masquerade, this is called diablerie and considered the most serious crime a vampire can commit. Notably, it's also a form of Cannibalism Superpower which requires eating the soul of the target.
 * Dungeons and Dragons
 * In Birthright some of the raw bloodline strength, sometimes along with abilities, can be stolen by killing its carrier in specific ways, though the killer should have at least some of divine touch to begin with. Blood Abominations tend to be very, very powerful because by the time people knows of them, they usually already have slaughtered lots of "blooded" folk and chose bloodtheft as the road to power.
 * Al Qadim has a special sort of wizards -- Jackals, who don't memorize spells on their own, but instead steal from other magic-users.

Web Comics

 * Yuuki from Sparkling Generation Valkyrie Yuuki is able to steal power from Otsana and Shebi

Web Original

 * In Red vs. Blue, the Meta steals the AIs and armor modifications of his fellow Freelancers, which provide the corresponding abilities to their Freelancers.
 * Gets even worse in 9KnackeredSouls (A crossover fic between Red vs. Blue and My Little Pony Friendship Is Magic) when it gets revealed he can drain the magical power of unicorns to restore himself and possibly even the life force of other ponies. As if he wasn't hard enough to take down already.

Western Animation

 * Van Kleiss from Generator Rex after being DePowered by Rex, steals his Nanites, temporarily turning him in normal boy. He gets power opposite to Rex's - creating EVO.
 * Despite being the Trope Namer for Mega Manning, the Ruby-Spears adaptation of Mega Man showed the Blue Bomber stealing the weapons of Robot Masters by touch, usually leaving them unable to use the same.
 * Evil Counterpart Kevin 11 from Ben 10 had this as his special ability, but never really took full advantage of it (except in an alternate future where he used it to become a combination of Sylar and Naraku) and even stopped using it entirely after his Heel Face Turn. Kevin's own evil counterpart, Aggregor, has the same ability and no qualms about using it.
 * Leech from Static Shock, who drained the powers of both Static and several members of the Rogues Gallery.