Sliver Gravemother

Each token must attack the appropriate player if able.

Exiling the card with encore is a cost to activate the ability. Once you announce that you're activating it, no player may take actions until you've finished. They can't try to remove the card from your graveyard to stop you from paying the cost.

If an effect stops a token from attacking a specific player, that token can attack any player, planeswalker, or battle, or not attack at all. If the effect stops the token from attacking a specific player unless a cost is paid, you don't have to pay that cost unless you want to attack that player.

If one of the tokens can't attack for any reason (such as being tapped), then it doesn't attack. If there's a cost associated with having it attack, you aren't forced to pay that cost, so it doesn't have to attack in that case either.

If one of the tokens somehow is under another player's control as the delayed triggered ability resolves, you can't sacrifice that token. It remains on the battlefield indefinitely, even if you regain control of it later.

If you control more than one legendary Sliver with the same name and the "legend rule" begins applying to them again (perhaps because Sliver Gravemother leaves the battlefield) you'll immediately have to comply with the rule and put all but one of those Slivers into the graveyard.

Opponents who have left the game aren't counted when determining how many tokens to create.

The "legend rule" is the rule that states that if a player controls two or more legendary permanents with the same name, that player chooses one of them, and the rest are put into their owners' graveyards.

The tokens copy only what's on the original card. Effects that modified that creature when it was previously on the battlefield won't be copied.

While the "legend rule" doesn't apply to Slivers you control, you can control any number of legendary Slivers with the same name and none of them will be put into the graveyard.