Which civilizations have the Assassins card?

French, Russians, Spanish, Swedes, Italians, Maltese, Poles and Mexicans.

What does Assassins do in AoE3?

Spy: damage +100%, damage bonus against Mercenary +5, damage bonus against Hero +5, damage bonus against Merc Type1 +5; Ships 1 Spanish Treasure Chest. The game says: "Spies become Assassins, which do more damage to normal units and vastly more damage to heroes and mercenaries. Unlocks a charged rifle attack that allows seeing the location of recently hit targets."

Home City card

Assassins

Assassins (tech ID HCXPAssassins) is a Home City card in AoE3: Definitive Edition 100.19.18309, in the decks of 8 civilizations, sent from the Colonial Age and Fortress Age. Effect: Spy: damage +100%, damage bonus against Mercenary +5, damage bonus against Hero +5, damage bonus against Merc Type1 +5; Ships 1 Spanish Treasure Chest.

Quick answers (3)

Which civilizations have the Assassins card?: French, Russians, Spanish, Swedes, Italians, Maltese, Poles and Mexicans.
What does Assassins do in AoE3?: Spy: damage +100%, damage bonus against Mercenary +5, damage bonus against Hero +5, damage bonus against Merc Type1 +5; Ships 1 Spanish Treasure Chest. The game says: "Spies become Assassins, which do more damage to normal units and vastly more damage to heroes and mercenaries. Unlocks a charged rifle attack that allows seeing the location of recently hit targets."
What is the tech ID of Assassins?: HCXPAssassins.

Spies become Assassins, which do more damage to normal units and vastly more damage to heroes and mercenaries. Unlocks a charged rifle attack that allows seeing the location of recently hit targets.

What it does

Spy: damage +100%, damage bonus against Mercenary +5, damage bonus against Hero +5, damage bonus against Merc Type1 +5
Ships 1 Spanish Treasure Chest(unit)