"They encircled our patrol with the stealth of snakes, corralling us like livestock."
—Merrik Aidar, Benalish patrol

Sidewinder Sliver

All Sliver creatures have flanking. (Whenever a creature without flanking blocks a Sliver, the blocking creature gets -1/-1 until end of turn.)

Secret Lair Drop

Time Spiral Remastered

Time Spiral

All Slivers have flanking. (Whenever a creature without flanking blocks a Sliver, the blocking creature gets -1/-1 until end of turn.)

Abilities that Slivers grant, as well as power/toughness boosts, are cumulative. However, for some abilities, like flying, having more than one instance of the ability doesn't provide any additional benefit.

If the creature type of a Sliver changes so it's no longer a Sliver, it will no longer be affected by its own ability. Its ability will continue to affect other Sliver creatures.

Once a Sliver has become blocked by a creature without flanking, the blocking creature gets -1/-1 even if the blocked Sliver loses flanking before the ability resolves.