Sliver of the Plaguelord

When Sliver of the Plaguelord enters the battlefield you may sacrifice a Sliver. If you do, target player gets two poison counters.
All Slivers have wither. (This deals damage to creatures in the form of -1/-1 counters.)
"These new Slivers are different. They are less Sliver like, and more like beings. They aren't themselves." - Karn, Liberated