Good afternoon AFK, Margery mastery has been abit of an off skill since the pub landed, at first it was broken and made the pet do nothing much but run away and cast spells.. then they change it... which made for ideal PvP pets... as the pet would use teleports to catch what ever it was chasing and spam mind blast if they were out of range...( sadly they nerfed it in PvP last week making is a lot less effective) .. IN Pvm(player vs monsters) Until they do some more fine tuning on it I would avoid it... Turning it on changes the UI of the pet.. it will use teleport a lot more(they did a small fix to it in the PvP nerf which helped this but still does it during very bad time which during spawns normally ends in pet death) It gains a few new spells, and tends to use mind blast, arc cure more often, and gains the player mage mastery move death beam(which is not bad if your pet is not the one tanking as it will stand in place and fire off damage of 100-150 ever sec until its mana is gone..but any damage breaks this effect).. but the one things which should make any player avoid mage mastery on pets(unless it a PvP pet) is this... The pet cast spell reflect Lowering its phy resist to 60 allowing even greater dragons to melee hit the pet for 60-90... and effectly ruining the pet for most solo PvM fights as it makes it take the worse spike I ever seen in test pets...requiring a much heavier lvl of tamer vetting, spell healing and player focus to keep it alive in heavy monster areas... bare in mind that this has just been my take on the pets Ive tested it on and others playstyle may vary and reflect other things... but I hope this helped mate... should ya have any other questions feel free to post or message me. =^-^=