"Wait, where am I? Who am I? And what are you?"
—Skyshroud secretkeeper, last words

Mesmeric Sliver

All Slivers have "When this permanent enters, you may fateseal 1." (To fateseal 1, its controller looks at the top card of an opponent's library, then they may put that card on the bottom of that library.)

Secret Lair Drop

Future Sight

All Slivers have "When this permanent comes into play, you may fateseal 1." (Its controller looks at the top card of an opponent's library, then he or she may put that card on the bottom of that library.)

This ability triggers when Mesmeric Sliver itself enters.

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.