If a creature spell's sneak cost was paid, the creature it becomes enters tapped and attacking the same player, planeswalker, or battle as the creature that was returned to its owner's hand to pay its sneak cost. This is a rule specific to sneak; in other cases, when a creature is put onto the battlefield attacking, that creature's controller chooses which player, planeswalker, or battle it's attacking.
Although a creature put onto the battlefield this way is attacking, it was never declared as an attacking creature. Abilities that trigger whenever a creature attacks won't trigger when that creature enters attacking.
If Shark Shredder deals combat damage to a player during the first strike combat damage step and its last ability returns a creature with first strike, the returned creature will still deal combat damage during the regular combat damage step. If the returned creature has double strike, it will deal combat damage only once during that regular combat damage step.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Adam Volker
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In the case where Shark Shredder deals combat damage to a player during the regular combat damage step (probably because another effect granted Shark Shredder double strike), the creature returned with Shark Shredder's last ability won't deal combat damage at all during that combat phase.
Spells can only be cast for their sneak costs any time you could play an instant during the declare blockers step on your turn (after your opponent has decided whether to block).
