Mob appearing

add a new procedure mod element

set the trigger to when player spawns

set player's nbt number tag to 0 (give the nbt tag your own name, something like "herobrineSpawnTimer" or something else)

then add a new procedure

set the trigger to loaded entity tick update

if player nbt tag is 6000 ticks (5 minutes), then 

if random number is less than a certain threshold, then

spawn herobrine

otherwise, set nbt tag to nbt tag + 1