to create a random item you have to use arrays.
Put that into the initialization
Item[0] = Blades of Attack
Item[1] = Ring of Protection
Item[2] = Mask of Death
Item[3] = Belt of giant Strength
And now you create a trigger like LorDz said, just add some things.
Event - A unit dies
Condition - Unit-type of triggering unit equal to (your unit) == true
Action -
Set TempPoint = Position of (triggering unit)
Set TempInt = Random integer number between (0 and 3)
Create 1 Item[TempInt] at TempPoint
Custom script: call RemoveLocation (udg_TempPoint)
EDIT:
To have a different drop chance for each item, just change this in the initialization trigger:
Item[0] = Blades of Attack
Item[1] = Blades of Attack
Item[2] = Mask of Death
Item[3] = Belt of giant Strength
now - cause there are 2 blades of attack - it has a 50% chance to drop while the other items have a 25% chance