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.
Mindwhip Sliver
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.
If you cast this as normal during your main phase, it will enter the battlefield and you’ll receive priority. If no abilities trigger because of this, you can activate its ability immediately, before any other player has a chance to remove it from the battlefield.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Jeff Miracola
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
