OnPlayerDamage
From the Scripting Manual
function OnPlayerDamage(Victim, Shooter: byte; Damage: integer): integer; Parameter Info: Victim (Byte): Player ID of the player being damaged. Shooter (Byte): Player ID of the player who is doing damage Damage (Integer): Amount of damage being done. Description: This procedure will be called every time a playe loses health, you can also change the amount of damage being done by changing the result value.
