I think you need to give more info on this Reaper
In your initial post it sounds like its a seperate attack and when it crits then we get the pet. Is this or is this not the case? Because adding a new crit mechanic for this game would be cool, i just dont know if summoning a whole new pet with it would work well (knowing you guys no offense)
based on the description, it is an attack skill at 20m range
when you crit you summon the reaper which lasts for a short period of time and smacks the crap out of the enemy
sounds interesting, the cooldown should be around the same as the duration of the summon
most lvl 90 players are running 100% crit so perma summon
doesn't work too well for lower level players or non-cashers