Create a variable integer called Random to make the item be created randomly whenever a creep dies.
Event-
Unit Dies
Condition-
Owner of unit equal to Neutral Hostile
Action-
Set Random equal to random integer between 1 to 3
If- Random equal to 1 then create tome of power at position of triggering unit else do nothing.
If- Random equal to 2 then do nothing else do nothing.
If- Random equal to 3 then do nothing else do nothing.
This trigger enables the tome of power to be dropped by a creep one-third of the time whenever it dies.
Sorry for the inaccuracy of the trigger but i hope this helps