The first wave of slivers perished from the Riptide wizards' magic. The second wave shrugged off their spells like water.

Ward Sliver

As this creature enters, choose a color.
All Slivers have protection from the chosen color.

The List

Secret Lair Drop

Legions

As Ward Sliver comes into play, choose a color.
All Slivers have protection from the chosen color.

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.