Chatras is a technology in Age of Empires II HD: Rise of the Rajas that is unique to the Vietnamese and can be researched at the Castle. Once researched, it gives +30 HP to Battle Elephants.
Strategy
The value of Chatras is that it makes the Battle Elephants highly resilient, and for a civilization like Vietnamese that requires support for their archers from other troops, having a tanky unit is a great advantage, as Battle Elephants can act as meat shields that guards the ranged units. A fully upgraded Elite Battle Elephant with both Bloodlines and Chatras researched will have 350 HP, a value that is only beaten by the War Elephant (620 HP) and rivaled by the Elephant Archer (350 HP).