Muscle Sliver
Tempest — Common
Creature — Sliver
Muscle Sliver rulings:
2021-03-19: Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to a Sliver may become lethal if Muscle Sliver leaves the battlefield during that turn.
2021-03-19: Muscle Sliver's ability does give itself +1/+1.
2013-07-01: 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.
2013-07-01: 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.
