Which civilizations have the Loyal Warriors card?

Ottomans, Portuguese, Italians and Maltese.

What does Loyal Warriors do in AoE3?

Native Warrior: Food cost −15%, Wood cost −15%, Coin cost −15%; Mercenary: Coin cost −15%; Outlaw: Coin cost −15%, Food cost −15%. The game says: "Native Warriors, Mercenaries and Outlaws are 15% cheaper, including shipments of them."

Home City card

Loyal Warriors

Loyal Warriors (tech ID DEHCREVLoyalWarriors) is a Home City card in AoE3: Definitive Edition 100.19.18309, in the decks of 4 civilizations, sent from the Fortress Age. Effect: Native Warrior: Food cost −15%, Wood cost −15%, Coin cost −15%; Mercenary: Coin cost −15%; Outlaw: Coin cost −15%, Food cost −15%.

Quick answers (3)

Which civilizations have the Loyal Warriors card?: Ottomans, Portuguese, Italians and Maltese.
What does Loyal Warriors do in AoE3?: Native Warrior: Food cost −15%, Wood cost −15%, Coin cost −15%; Mercenary: Coin cost −15%; Outlaw: Coin cost −15%, Food cost −15%. The game says: "Native Warriors, Mercenaries and Outlaws are 15% cheaper, including shipments of them."
What is the tech ID of Loyal Warriors?: DEHCREVLoyalWarriors.

Native Warriors, Mercenaries and Outlaws are 15% cheaper, including shipments of them.

What it does

Native Warrior: Food cost −15%, Wood cost −15%, Coin cost −15%
Mercenary: Coin cost −15%
Outlaw: Coin cost −15%, Food cost −15%

3 further effects (interface, visuals or AI) not listed.

Cost: 1
Sent from: III
Times it can be sent: 1
Game ID: DEHCREVLoyalWarriors

In these decks

Source: techtreey.xml and the Home City files, AoE3: DE 100.19.18309 (build 25040513).