Can I continue to look at my opponent's hand throughout the entire turn with Scuttle Crab Deathknell? So if he plays a secret after the Deathknell trigger, can I re-look at his hand?
No, you cannot continue to look at your opponent's hand throughout the entire turn following the resolution of Scuttle Crab's Deathknell effect.
According to Rule 424.3 and RiftJudge FAQs #9700 and #10164, revealing an opponent's hand is a one-time, temporary action. The reveal only lasts as long as it takes for the players present to clearly read and understand the information in the hand. Once that information is understood, the reveal action is complete, and the opponent's hand returns to being private information. It does not stay revealed for the remainder of the turn, and you cannot re-examine it later if they draw or play new cards.
Regarding your question about "facedown cards," Scuttle Crab's Deathknell ability explicitly contains two separate effects:
- "They reveal their hand." (This is the temporary reveal described above).
- "You can look at their facedown cards this turn."
While the hand reveal is temporary, the second part of the ability specifically grants you permission to look at your opponent's facedown cards (such as Hidden units or gear) for the remainder of that turn. This allows you to check those specific cards at any time until the turn ends, even if the opponent plays new facedown cards later in the same turn.
Summary:
- Opponent's Hand: You only see it once, briefly, upon the resolution of the Deathknell. You cannot re-look at their hand later if they draw new cards.
- Opponent's Facedown Cards: Because the card text explicitly grants you this permission "this turn," you may continue to look at any of their facedown cards on the battlefield at any time for the remainder of that turn.
Note: I could not fully verify the interpretation regarding the "facedown cards" portion of the effect against the retrieved rules.