Darkheart Sliver — ruling 1

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.

Darkheart Sliver — ruling 2

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.

Darkheart Sliver — ruling 3

The player who controlled the Sliver that's sacrificed gains the life, not the controller of Darkheart Sliver.

Darkheart Sliver : règles, fonctionnement et interactions (MTG)

Darkheart Sliver is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

All Slivers have "Sacrifice this permanent: You gain 3 life."

How it works : Darkheart Sliver

Original English text — Wizards of the Coast / Scryfall

#1July 1, 2013· Wizards of the Coast
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.
No other card is named in this ruling.
#2July 1, 2013· Wizards of the Coast
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.
No other card is named in this ruling.
#3February 1, 2007· Wizards of the Coast
The player who controlled the Sliver that's sacrificed gains the life, not the controller of Darkheart Sliver.
No other card is named in this ruling.