Zareth San, the Trickster

• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2020-09-25
  Official Ruling
  You choose which player or planeswalker Zareth San is attacking. It doesn't have to be attacking the same player or planeswalker that the returned Rogue was attacking.
• WWizards of the Coast2020-09-25
  Official Ruling
  Before blockers are declared, attacking Rogues are neither blocked nor unblocked, so you can't pay the cost to activate Zareth San's second ability.
• WWizards of the Coast2020-09-25
  Official Ruling
  Although Zareth San is attacking, it was never declared as an attacking creature (for purposes of abilities that trigger whenever a creature attacks, for example).
• WWizards of the Coast2020-09-25
  Official Ruling
  To activate Zareth San's second ability, you reveal it in your hand. It remains revealed until the ability has resolved.
• WWizards of the Coast2020-09-25
  Official Ruling
  Zareth San's second ability can be activated during the declare blockers step, combat damage step, or end of combat step. If you wait until after the declare blockers step, because all combat damage is dealt at once, Zareth San won't normally deal combat damage.
• WWizards of the Coast2020-09-25
  Official Ruling
  If Zareth San leaves your hand before its second ability resolves, the ability does nothing as it resolves. Zareth San remains in its new zone and the Rogue you returned remains in its owner's hand.
• WWizards of the Coast2020-09-25
  Official Ruling
  If a creature in combat has first strike or double strike, you can activate Zareth San's second ability during the first-strike combat damage step. Zareth San will deal combat damage during the regular combat damage step in this case, even if it somehow gains first strike.
• WWizards of the Coast2020-09-25
  Official Ruling
  In a multiplayer game, if a player leaves the game, all cards that player owns leave as well. If you leave the game, any permanents you control from Zareth San's last ability are exiled.

Rulings via Scryfall