Slivers
The Slivers are a race of creatures in Magic: The Gathering. Renowned for their adaptability, fast breeding and hive-like social structure, they are a menace to deal with both in the fluff and crunch. Slivers haver serpentine bodies approximately five meters long and wedge-like heads that aside from their mouths are featureless. A Sliver's tail is forked, where this fork begins differs depending on the individual species of Sliver and can be any length between the last 10% to two-thirds of its body. The bodies themselves are little more than sinew and chitin for the hard parts, combined with a powerful digestive tract to quickly devour anything they encounter, returning this mass to the Hive to strengthen it and grow new Slivers, much like the Tyranids.
The most notable feature of the Slivers is how they convey their abilities onto nearby Slivers. So when a Sliver with wings or an enhanced carapace joins a group that doesn't have these traits they will quickly grow these traits of their own. This means that even a small group of Slivers with a diverse number of traits can rapidly become a force to be reckoned with, each being a sum of the brood's abilities.
The obvious downside (and blessing, depending on who you ask) is that if you kill a Sliver those around it lose whatever trait it bestowed on its allies. So by eliminating specific Slivers within a brood the others become less and less dangerous with every death until they are easily overcome.
In-game, Slivers appear evenly in all colors, mostly granting abilities that fall within the various parts of the color pie. They have low casting costs (many are only 4 or less CMC) and they all require only one mana within their color, adding to their castability.
Appearances
Thus far, the Slivers appeared in four different blocks. The first three times they appeared spread out over the blocks, the fourth time they appeared in a Core set.
Tempest Cycle
In the Tempest Cycle the Slivers appeared for the first time, spread over the first two blocks. In Tempest there were eleven Slivers: five common 1/1s with a CMC of 2 and five uncommon 2/2s with a CMC of 3, one for each color. Five of these Slivers have passive abilities, five have activated abilities that cost 2. The eleventh Sliver is Metallic Sliver, a 1/1 artifact creature for 1.
Stronghold had the other six Slivers in the cycle: five uncommon two-colored 2/2s for 2. The sixth is the Sliver Queen, one of three Legendary Slivers. She can generate 1/1 Sliver tokens for 2, a fine deal later in the game to bolster your ranks.
Onslaught Cycle
Returning in Onslaught Cycle, there are sixteen Slivers. The first fifteen appear in Legions: five common 1/1s, five uncommon 2/2s and five rare 3/3s, one for each color. Some of the most powerful Slivers are in this batch; Shifting Sliver, Toxin Sliver and Ward Sliver are all but obligations for decks that can use them.
The sixteenth Sliver is in Scourge: the Sliver Overlord who allows you to search your Deck for Slivers or take control of your opponent's Slivers.
Time Spiral
The Time Spiral cycle contains by far the most Slivers. The first set, Time Spiral, contains 26 Slivers. There are two commons, one uncommon and one rare per color, a cycle of allied-color uncommons and an overpriced artifact creature who is 5 for a 3/3.
The Slivers from Planar Chaos are... weird. Eleven Slivers this time around: five overpriced common monocolored Slivers with abilities outside of the Color Pie, five uncommon enemy-colored two-colored 2/2s and a White time-shifted Sliver that grants +1/+1 to your Slivers.
Future Sight introduced six more Slivers: five Future Shifted cards with then-unseen abilities and the third Legendary Sliver: the Sliver Legion who buffs every Sliver on the battlefield: +1/+1 for every Sliver in play.
Magic 2014
Like the fourth iteration of another of Wizards of the Coast's products, they cocked up the fourth batch of Slivers. Big time. Not only was the traditional "one Sliver per color" fucked right in the ass, they fucked up the designs of the Slivers themselves the most. Gone are the murderworms of yore, now they are some kind of creature best described as a Tyrannoformed Predator. They have gone to a bipedal form with long head-tentacles, clawed arms with sometimes the old-school talons, legs with three joints and chitinous bodies. While the design itself is not bad, the fact that they replaced the true Slivers is several flavors of bullshit.
There are 14 Slivers in this set: five commons (two Red, two Green and one White), three uncommons (Red, Green and White), five Rares (one for each color) and an artifact creature who is 3 for 2/2. Not a lot of new things are added to the table: most are reprints of previous Slivers, albeit with a different cost.
The only interesting thing about this is that these Slivers have build their hive on the rogue plane Shandalar, which should raise some eyebrows (and perhaps boners) amongst the veterans of the game.