2lets keep this simple the rules are 

each person says something and it happens,apears,or is summond but you have to role the more rediculess the idea the higher the dice roll need to be

also it must be reasonable and if they fail you must say the number it was

example:

player one:a goblin apears         player three: a horde of mind flares appear 

player two:role                             player four: roll

player one: 12                               player three:12 

player two:sucess                        player four: fail it was 18